Puff of smoke (4)
I believe the answer is:
wisp
'puff 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 "Something insubstantial" , "Thin strand, of smoke say" , "Small thin line of cloud or smoke" , "whose will is so elusive?" , "Slight trace" .)