Lay bare in sun's heat, heating (9)
I believe the answer is:
unsheathe
'lay bare' is the definition.
(thesaurus)
'in sun's heat heating' is the wordplay.
'in' indicates a hidden word.
'UNSHEATHE' is hidden in the letters of 'suns heat heating'.
(Other definitions for unsheathe that I've seen before include "Pull out (a blade)" , "Take out of scabbard" , "Remove from cover like sword" , "Draw from a scabbard" , "Draw (a sword)" .)