Be malodorous (4)
I believe the answer is:
reek
'be malodorous' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reek that I've seen before include "smoke in Balmoral" , "bad odour" , "Smell foully" , "Smell like 12 Across" , "hum a lot!" .)