Smell (unpleasantly) (4)
I believe the answer is:
pong
'smell unpleasantly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pong that I've seen before include "Niff" , "It may make one wrinkle nose" , "In slang, a real stink" , "Ping's regular partner?" , "Horrid smell (informal)" .)