Thin trail of smoke (4)
I believe the answer is:
wisp
'thin trail of smoke' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wisp that I've seen before include "Small streak of smoke or cloud" , "Gauzy strand" , "Small thin bunch/strand (of something)" , "Thin fine strand" , "Something insubstantial" .)