Cover protects doctor in struggle (6)
I believe the answer is:
combat
'struggle' is the definition.
(I know that combat is a more specific form of contend)
'cover protects doctor' is the wordplay.
'cover' becomes 'coat' (coating is a kind of covering).
'protects' indicates putting letters inside (outer letters protect inside letters).
'doctor' becomes 'MB' (Bachelor of Medicine).
'coat' enclosing 'mb' is 'COMBAT'.
'in' is the link.
(Other definitions for combat that I've seen before include "A fight or contest, sometimes mortal" , "Military fighting" , "Fight against; fighting" , "Fight or contest, may be mortal" , "sort of jacket" .)