It's nonsense making a pig clean (7)
I believe the answer is:
hogwash
'it's nonsense' is the definition.
(I've seen this before)
'a pig clean' is the wordplay.
'a pig' becomes 'hog' ('hog' can be a synonym of 'pig').
'clean' becomes 'wash' (washing is a kind of cleaning).
'hog'+'wash'='HOGWASH'
'making' acts as a link.
(Other definitions for hogwash that I've seen before include "Balderdash - pigswill" , "Utter nonsense" , "Utter rubbish" , "Bunkum" , "garbage" .)