Uncompromising firm's leading chain (8)
I believe the answer is:
hardline
'uncompromising' is the definition.
(I've seen this before)
'firm's leading chain' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'leading' says to put letters next to each other (some letters go in front of others).
'chain' becomes 'line' (a line or chain of things).
'hard'+'line'='HARDLINE'
(Other definitions for hardline that I've seen before include "uncompromising" , "Tough policy" .)