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

Ross

I believe the answer is:

unarm

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'render harmless' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(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 Sunday Times and The Times.
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