Oi! At mint I thwarted forgery! (9)
I believe the answer is:
imitation
'forgery' is the definition.
(synonyms)
'oi at mint i thwarted' is the wordplay.
'thwarted' indicates anagramming the letters.
'oi'+'at'+'mint'+'i'='oiatminti'
'oiatminti' is an anagram of 'IMITATION'.
(Other definitions for imitation that I've seen before include "Simulated" , "It's said to be the sincerest form of flattery" , "Unoriginal" , "Ersatz, fake" , "Act of copying" .)