Moonie's bum is foul-smelling (7)
I believe the answer is:
noisome
'foul-smelling' is the definition.
(synonyms)
'moonie's bum' is the wordplay.
'bum' indicates anagramming the letters (bum can mean faulty or wrong).
'moonies' anagrammed gives 'NOISOME'.
'is' acts as a link.
(Other definitions for noisome that I've seen before include "Stinking" , "Offensive, noxious" , "Rank" , "Evil-smelling - sounds loud" , "noxious smells" .)