Not clean (5)
I believe the answer is:
dirty
'not clean' is the definition.
('dirty' is the opposite to 'clean')
This is all the clue.
(Other definitions for dirty that I've seen before include "Unclean or indecent" , "Unscrupulous" , "1971 film with Clint Eastwood" , "Salacious" , "Not clean" .)