Rising current of warm air (7)
I believe the answer is:
thermal
'rising current of warm air' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for thermal that I've seen before include "which can take a glider up" , "Designed to retain heat" , "Upward current of warm air" , "Up-current of air" , "Warm like winter woollies" .)