I stand up to a Greek character (4)

I believe the answer is:

iota

'greek character' is the definition.
(letter from the Greek alphabet)

'i stand up to a' is the wordplay.
'stand up' shows that the letters should be reversed in order (in a down clue, the word stands upside down).
'to' reversed gives 'ot'.
'i'+'ot'+'a'='IOTA'

(Other definitions for iota that I've seen before include "Just a jot, from Greek alphabet" , "Jot, particle" , "Scintilla" , "Grain" , "little part" .)

I've seen this clue in The Telegraph.

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