The one thing lacking is the Lord's own gift, which the rich man could not add
Clement is interpreting Jesus' words to the rich man in the Gospel, explaining that the man was not yet perfect and that the one thing he lacked was the gift of eternal life, which he failed to receive because he did not truly will it.
"If you wish to be perfect." So he was not yet perfect, for nothing is more perfect than the perfect. And the divine "if you wish" revealed the free will of the soul conversing with him. The choice belonged to the man, as a free being; the gift belonged to God, as Lord. He gives to those who are willing, who are exceedingly eager, who ask, so that in this way their salvation becomes their own. For God does not compel; force is hateful to God. He provides for those who seek, he gives to those who ask, and he opens to those who knock (Matthew 7:7). If you wish, then, if you truly wish and do not deceive yourself, acquire what is lacking. "One thing you lack" (Mark 10:21): the one thing, which is mine. The good. What is already beyond the law. What the law does not give. What the law cannot contain, what belongs to the living. Indeed, the man who fulfilled all the things of the law "from his youth" and boasted of his excessive accomplishments could not add this one thing to all the rest: the special gift of the Savior, so that he might receive the eternal life he longed for. But he went away vexed, disgusted by the command of life. For he did not truly want life, as he claimed; he was only clothing himself in the reputation of a good choice. He was capable of busying himself with many things, but the one thing, the work of life, he was unable, unwilling, and too weak to accomplish. It was like what the Savior said to Martha, who was busy with many things, being dragged around and troubled in her serving, and who complained about her sister for leaving the service and sitting at his feet in the leisure of a disciple: "You are troubled about many things; Mary has chosen the good part, and it will not be taken from her" (Luke 10:41-42). So also he commanded this man to let go of his restless busyness and cling to the one thing and sit beside it, the grace of the one who grants eternal life.
Greek
»Εἴ θέλεις τέλειος γενέσθαι.« οὐκ ἄρα πω τέλειος ἦν· οὐδὲν γὰρ τελείου τελειότερον. καὶ θείως τὸ »εἰ θέλεις« τὸ αὐτεξούσιον τῆς προσδιαλεγομένης αὐτῷ ψυχῆς ἐδήλωσεν. ἐπὶ τῷ ἀνθρώπῳ γὰρ ἦν ἡ αἵρεσις ὡς ἐλευθέρῳ, ἐπὶ θεῷ δὲ ἡ δόσις ὡς κυρίῳ. δίδωσι δὲ βουλομένοις καὶ ὑπερεσπουδακόσι καὶ δεομένοις, ἵν᾿ οὕτως ἴδιον αὐτῶν ἡ σωτηρία γένηται. οὐ γὰρ ἀναγκάζει ὁ θεός, βία γὰρ ἐχθρὸν θεῷ, ἀλλὰ τοῖς ζητοῦσι πορίζει καὶ τοῖς αἰτοῦσι παρέχει καὶ τοῖς κρούουσιν ἀνοίγει. εἰ θέλεις οὖν, εἰ ὄντως θέλεις καὶ μὴ ἑαυτὸν ἐξαπατᾷς, κτῆσαι τὸ ἐνδέον. »ἕν σοι λείπει«, τὸ ἕν, τὸ ἐμόν. τὸ ἀγαθόν. τὸ ἤδη ὑπὲρ νόμον. ὅπερ νόμος οὐ δίδωσιν. ὅπερ νόμος οὐ χωρεῖ, ὃ τῶν ζώντων ἴδιόν ἐστιν. ἀμέλει ὁ πάντα τὰ τοῦ νόμου πληρώσας »ἐκ νεότητος« καὶ τὰ ὑπέρογκα φρυαξάμενος τὸ ἓν τοῦτο προσθεῖναι τοῖς ὅλοις οὐ δεδύνηται. τὸ τοῦ σωτῆρος ἐξαίρετον, ἵνα λάβῃ ζωὴν αἰώνιον, ἣν ποθεῖ· ἀλλὰ δυσχεράνας ἀπῆλθεν, ἀχθεσθεὶς τῷ παραγγέλματι τῆς ζωῆς. ὑπὲρ ἧς ἱκέτευεν. οὐ γὰρ ἀληθῶς ζωὴν ἤθελεν, ὡς ἔφασκεν, ἀλλὰ δόξαν προαιρέσεως ἀγαθῆς μόνην περιεβάλλετο. καὶ περὶ πολλὰ μὲν οἷός τε ἧν ἀσχολεῖσθαι, τὸ δὲ ἕν, τὸ τῆς ζωῆς ἔργον, ἀδύνατος καὶ ἀπρόθυμος καὶ ἀσθενὴς ἐκτελεῖν· ὁποῖόν τι καὶ πρὸς τὴν Μάρθαν εἶπεν ὁ σωτὴρ ἀσχολουμένην περὶ πολλὰ καὶ περιελκομένην καὶ ταρασσομένην διακονικῶς, τὴν δὲ ἀδελφὴν αἰτιωμένην, ὅτι τὸ ὑπηρετεῖν ἀπολιποῦσα τοῖς ποσὶν αὐτοῦ παρακάθηται μαθητικὴν ἄγουσα σχολήν· »σὺ περὶ πολλὰ ταράσσῃ· Μαρία δὲ τὴν ἀγαθὴν μερίδα ἐξελέξατο, καὶ οὐκ ἀφαιρεθήσεται αὐτῆς.« οὕτως καὶ τοῦτον ἐκέλευε τῆς πολυπραγμοσύνης ἀφέμενον ἑνὶ προστετηκέναι καὶ προσκαθέζεσθαι, τῇ χάριτι τοῦ ζωὴν αἰώνιον προστιθέντος.