Pig clean? Nonsense! (7)
I believe the answer is:
hogwash
'nonsense' is the definition.
(I've seen this before)
'pig clean?' is the wordplay.
'pig' becomes 'hog' (synonyms).
'clean?' becomes 'wash' (washing is a kind of cleaning).
'hog'+'wash'='HOGWASH'
(Other definitions for hogwash that I've seen before include "garbage" , "Pigswill - nonsense!" , "Utter rubbish" , "Balderdash - pigswill" , "Baloney" .)