Smell starts putting off new gardeners (4)
I believe the answer is:
pong
'smell' is the definition.
(pong can informally mean a bad smell)
'starts putting off new gardeners' is the wordplay.
'starts' indicates taking the first letters.
The first letters of 'putting off new gardeners' is 'PONG'.
(Other definitions for pong that I've seen before include "Horrid smell (informal)" , "Niff" , "Bad smell" , "Smell unpleasantly" , "Ping's regular partner?" .)