Starts to produce odious, noxious gassy stench (4)
I believe the answer is:
pong
'stench' is the definition.
(I've seen this before)
'starts to produce odious noxious gassy' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'produce odious noxious gassy' is 'PONG'.
(Other definitions for pong that I've seen before include "Ping's regular partner?" , "hum" , "Niff" , "Smell unpleasantly" , "Horrid smell (informal)" .)