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