In this passage, that’s imperative aorist active 2nd person singular from προσέχω.
It’s a short form for πρόσχες μοι <τὸν νοῦν>: pay attention to me.
We would expect πρόσσεχες, as in Plato’s Theaetetus (198b): Τῷ δὲ δὴ ἐντεῦθεν ἤδη πρόσσχες τὸν νοῦν.
But the form πρόσχες appears many times in different authors and especially in the LXX.
Hope this helps.
@tico - Yes thanks that’s what I had suspected because Muraoka in his lexicon entry for προσεχω lists προσσχες as the imperative but adds “On the spelling with -σσ- see Walters, The Text of the Septuagint: its Corruptions and their Emendations” but unfortunately I don’t have access to that work.
An interesting morphological detail: since ἔχω has a second thematic aorist in the other moods (indicative: ἔσχον etc.), we would expect the imperative σχέ (like βάλε, εἰπέ, etc.) and not σχέ-ς. It seems to follow the athematic imperative we find in θές (τίθημι), δός (δίδωμι), ἕς (ἵημι).