Curl of a cloud (4)
I believe the answer is:
wisp
'curl of a cloud' 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 "Thin girl" , "Small tuft of hair" , "Something insubstantial" , "Small streak of smoke or cloud" , "Small thin line of cloud or smoke" .)