Peers settle heartless quarrels (6)
I believe the answer is:
equals
'peers' is the definition.
('equal' can be a synonym of 'peer')
'settle heartless quarrels' is the wordplay.
'settle' indicates anagramming the letters (I've seen 'settling' mean this (settle can mean to solve or sort out)).
'heartless' suggests removing the centre.
'quarrels' with its centre removed is 'quaels'.
'quaels' anagrammed gives 'EQUALS'.
(Other definitions for equals that I've seen before include "Peer group" , "They're at the same level" , "Those with the same status, rights, etc." , "Peers, matches" , "Corresponds to" .)