Gain by Labour unseating new Liberal peer (4)
I believe the answer is:
earl
'peer' is the definition.
(earl is a kind of peer)
'gain by labour unseating new liberal' is the wordplay.
'gain by labour' becomes 'earn' (to earn is to gain through work or labour).
'unseating' suggests deleting specific letters (to unseat is to throw out or oust).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'liberal' becomes 'l' (abbreviation).
'earn' with 'n' removed is 'ear'.
'ear'+'l'='EARL'
(Other definitions for earl that I've seen before include "Real kind of English lord" , "A real peer" , "British nobleman" , "Male member of British nobility" , "Countesss husband" .)