Caught in relentless heat, hire protective cover (6)
I believe the answer is:
sheath
'protective cover' is the definition.
(I know that sheath is a type of protective cover)
'caught in relentless heat hire' is the wordplay.
'caught in' indicates a hidden word.
'SHEATH' can be found hidden inside 'relentless heat hire'.
(Other definitions for sheath that I've seen before include "Sword covering" , "jacket" , "case for weapon" , "Cover for a blade" , "Covering for blade" .)