Smell coming from factories once around (4)
I believe the answer is:
nose
'smell' is the definition.
(I know that nose is a type of smell)
'coming from factories once around' is the wordplay.
'coming from' indicates the answer is hidden within the clue.
'around' shows that the letters should be reversed in order.
'eson' is hidden within 'factories once'.
'eson' backwards is 'NOSE'.
(Other definitions for nose that I've seen before include "scent" , "Organ of smell" , "Bouquet of wine; beak" , "Faculty for detecting" , "Nuzzle" .)