Say nothing about fever as protective cover (6)
I believe the answer is:
sheath
'protective cover' is the definition.
(I've seen this before)
'say nothing about fever' is the wordplay.
'say nothing' becomes 'sh' ('sh' is said to tell someone to say nothing).
'about' indicates putting letters inside.
'fever' becomes 'heat' (a fever might make you hot).
'sh' enclosing 'heat' is 'SHEATH'.
'as' acts as a link.
(Other definitions for sheath that I've seen before include "Case for a dagger" , "Protective covering for a knife" , "coat" , "Protective cover" , "Cover for a knife" .)