Cigarette around noon -- it's in the mouth (4)
I believe the answer is:
fang
'it's in the mouth' is the definition.
(fang can mean a sharp tooth)
'cigarette around noon' is the wordplay.
'cigarette' becomes 'fag' (synonyms).
'around' indicates putting letters inside.
'noon' becomes 'n'.
'fag' placed around 'n' is 'FANG'.
(Other definitions for fang that I've seen before include "Wolf's tooth" , "Long pointed tooth" , "Animal's sharp tooth" , "Large canine tooth" , "Poisonous tooth" .)