Mucky duck possibly picked up? (4)
I believe the answer is:
foul
'mucky' is the definition.
(thesaurus)
'duck possibly picked up?' is the wordplay.
'duck possibly' becomes 'fowl' (duck is a kind of fowl).
'picked up?' shows a homophone (sound like) (I've seen 'picked up' mean this).
'fowl' sounds like 'FOUL'.
(Other definitions for foul that I've seen before include "Offensive, abominable" , "Abhorrent" , "Unfair, against rules of game" , "Repulsive" , "Obnoxious" .)