Mon petit could be powerless (8)
I believe the answer is:
impotent
'powerless' is the definition.
(similar in meaning)
'mon petit could be' is the wordplay.
'could be' indicates anagramming the letters.
'mon'+'petit'='monpetit'
'monpetit' with letters rearranged gives 'IMPOTENT'.
(Other definitions for impotent that I've seen before include "Powerless like mon petit" , "Useless" , "Lacking power" , "Ineffective" , "Mon petit may be powerless" .)