Crossword Genius

A hundred euros initially, which is non-returnable (3)

Ross

I believe the answer is:

ace

'which is non-returnable' is the definition.
(tennis serve which cannot be returned)

'a hundred euros initially' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'initially' says to take the initial letters.
The initial letter of 'euros' is 'e'.
'a'+'c'+'e'='ACE'

(Other definitions for ace that I've seen before include "In golf, a hole in one" , "SERVE?] that's non-returnable" , "fighter pilot" , "Possibly a diamond" , "winner in court" .)

I've seen this clue in The Times.
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