Cover field where fighting takes place (5,4)
I believe the answer is:
front line
'where fighting takes place' is the definition.
'cover field' is the wordplay.
'cover' becomes 'front' (both can mean a deceptive facade).
'field' becomes 'line' (both can refer to an industry or area of expertise**).
'front'+'line'='FRONT LINE'
(Other definitions for front line that I've seen before include "Where the fighting takes place" , "Most advanced troops" , "sharp end" .)