Vehement cheat's in US it turned out (12)
I believe the answer is:
enthusiastic
'vehement' is the definition.
(thesaurus)
'cheat's in us it turned out' is the wordplay.
'turned out' indicates anagramming the letters.
'cheats'+'in'+'us'+'it'='cheatsinusit'
'cheatsinusit' anagrammed gives 'ENTHUSIASTIC'.
(Other definitions for enthusiastic that I've seen before include "Showing keen interest" , "Highly keen" , "Full of fervour" , "Eager" , "warm" .)