Keep on griping about the smell (4)
I believe the answer is:
pong
'the smell' is the definition.
(pong can informally mean a bad smell)
'keep on griping about' is the wordplay.
'about' indicates a hidden word (the clue goes around or about the answer).
'PONG' is hidden within 'keep on griping'.
(Other definitions for pong that I've seen before include "Reek" , "Niff" , "In slang, a real stink" , "Horrid smell (informal)" , "Bad smell" .)