Celtic love thrashing United (10)
I believe the answer is:
collective
'united' is the definition.
(associated in meaning)
'celtic love thrashing' is the wordplay.
'thrashing' indicates anagramming the letters (I've seen this in other clues).
'celtic'+'love'='celticlove'
'celticlove' is an anagram of 'COLLECTIVE'.
(Other definitions for collective that I've seen before include "Done by a group" , "body that works together" , "group working co- operatively" , "Done by people acting together" .)