Render harmless title and a Greek character turned up (5)

I believe the answer is:

unarm

I'm a little stuck... Click here to teach me more about this clue!

'render harmless' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for unarm that I've seen before include "Remove weapons from" , "Take weapon" , "Deprive of all weapons" , "Disable" , "Deprive of a weapon" .)

I've seen this clue in The Times and The Sunday Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.