Crossword Genius

Unions attack centrists' letter? (4)

Ross

I believe the answer is:

iota

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'letter?' is the definition.
(Greek letter)

I don't understand the rest of the clue.

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(Other definitions for iota that I've seen before include "Greek letter - small quantity" , "letter from abroad" , "Just a jot, like Greek letter" , "Smidgeon" , "(Not one) little bit" .)

I've seen this clue in The Irish Times.
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