A chap employed by a newspaper firm (7)
I believe the answer is:
adamant
'firm' is the definition.
(adamant can mean firm or determined)
'a chap employed by a newspaper' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'employed by' is an insertion indicator (inserted letters used or employed inside).
'newspaper' becomes 'DT' (Daily Telegraph).
'a'+'man'='aman'
'a'+'dt'='adt'
'aman' going into 'adt' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "Unshakable (in an opinion)" , "popstar" , "Refusing to be persuaded" , "Refusing to budge" , "Hard and unrelenting" .)