Setter aiding anagram break up (12)
I believe the answer is:
disintegrate
'break up' is the definition.
'setter aiding anagram' is the wordplay.
'anagram' indicates an anagram.
'setter'+'aiding'='setteraiding'
'setteraiding' is an anagram of 'DISINTEGRATE'.
(Other definitions for disintegrate that I've seen before include "Crumble" , "Lose all cohesion" , "Break into little pieces" , "decay" , "collapse" .)