Of heat (7)
I believe the answer is:
thermal
'of heat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for thermal that I've seen before include "Upward current of warm air" , "Warm or hot, say air or spring" , "which can take a glider up" , "type of baths" , "Column of rising air used by gliders" .)