Bit of smoke (4)
I believe the answer is:
wisp
'bit 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 "Gauzy strand" , "Small strand of hair etc" , "(Smoke) thin plume" , "whose will is so elusive?" , "Thin girl" .)