Cover fire in silence (6)
I believe the answer is:
sheath
'cover' is the definition.
(sheath is a kind of cover)
'fire in silence' is the wordplay.
'fire' becomes 'heat' (heating is a kind of firing**).
'in' means one lot of letters goes inside another.
'silence' becomes 'sh' (I've seen this in other clues).
'heat' going within 'sh' is 'SHEATH'.
(Other definitions for sheath that I've seen before include "case for weapon" , "(Blade's) cover" , "Protective cover" , "jacket" , "Close fitting dress" .)