Thin curl of smoke (4)
I believe the answer is:
wisp
'thin curl 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 streak of smoke or cloud" , "Delicate strand" , "Light streak of smoke or stray hair" , "Stray lock of hair" , "Slight trace" .)