Well, you could go to a morphological analyser of Ancient Greek, type in the word, and see what comes out. Such as morpheus on Perseus, or the other offshots of morpheus publicly available, or the subscription only variant of morpheus that I worked on for thirteen glorious years at the Thesaurus Linguae Graecae, before my contract was terminated last year.
Or… you could get a refresher from Dr Nick in how Ancient Greek verbs work.
The structure of Ancient Greek verbs is as follows—where * means optional:
PREVERB* AUGMENT* REDUPLICATION* ROOT TENSE-STEM-SUFFIX INFLECTION
The (preverb +) root is what you’re after in the dictionary.
The preverb is a chain of one or more prepositions, and you’ll recognise them pretty quickly. In this case, ὑφ- hyph-. Occasionally, what looks like a preposition is actually part of the verb, but not today. So you take off the hyph-; and because hyph– is what happens when hypo– precedes a word with a rough breathing, you leave the /h/ behind: ἡιρείτω hēireitō. Can I use the iota subscript please? ᾑρείτω.
You will have worked out that there is no verb hēireō in the dictionary. Now, a reduplication is a repetition of the first consonant of the root, followed by an /e/. That’s not the case here. So we try out hēi- being an augment. (And remember that the augment acts as the equivalent of the reduplication, if the verb starts with a vowel.)
The thing about long diphthongs, those things with iota subscripts, is that they are rare in Greek roots in general; and they are dead giveaways of augments on short diphthongs. So ᾑρείτω hēireitō is an augmented form of αἱρείτω haireitō. Yes, the long version of hai– should have been hāi-; but remember from your Greek historical linguistics: Proto-Greek ā regularly went in Ionic and Attic to ē.
So we’re left with hyph– + haire-. The verb you’re looking for is ὑφαιρέω ‘to take under = take away underhand, steal’. This is the imperative “let him steal”, although that ending looks present, not perfect…