Terrible lot, betrayal causing mêlée (6,5)
I believe the answer is:
battle royal
'melee' is the definition.
(synonyms)
'terrible lot betrayal' is the wordplay.
'terrible' is an anagram indicator.
'lot'+'betrayal'='lotbetrayal'
'lotbetrayal' anagrammed gives 'BATTLE ROYAL'.
'causing' is the link.
(Other definitions for battle royal that I've seen before include "long row" , "Fiercely contested fight" , "Fiercely-contested fight" , "Fiercely contested dispute" , "Fierce fight" .)