Sign of fire (5)
I believe the answer is:
smoke
'sign of fire' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smoke that I've seen before include "Cure (eg, herring)" , "Fume" , "Emit fumes" , "Cure (food) - endanger health!" , "Use a cigarette" .)