Get out of sun's heat, heading inside (9)
I believe the answer is:
unsheathe
'get out' is the definition.
(unsheathing is a kind of getting out)
'sun's heat heading inside' is the wordplay.
'inside' indicates a hidden word.
'UNSHEATHE' can be found hidden inside 'suns heat heading'.
'of' acts as a link.
(Other definitions for unsheathe that I've seen before include "Draw from a scabbard" , "Remove from cover like sword" , "Pull out (a blade)" , "Draw (a sword)" , "Draw out (a knife)" .)