Trace of smoke (4)
I believe the answer is:
wisp
'trace of smoke' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wisp that I've seen before include "Small thin line of cloud or smoke" , "Delicate strand" , "Stray lock of hair" , "whose will is so elusive?" , "Curling bit of smoke, hair, etc" .)