Mel's been rearranging group of musicians (8)
I believe the answer is:
ensemble
'group of musicians' is the definition.
(I've seen this before)
'mel's been rearranging' is the wordplay.
'rearranging' indicates an anagram (I've seen 'rearranged' mean this).
'mels'+'been'='melsbeen'
'melsbeen' is an anagram of 'ENSEMBLE'.
(Other definitions for ensemble that I've seen before include "Group of musicians or co-ordinated clothes" , "team" , "Set of matches" , "Group of musicians or a co-ordinated outfit" , "Group of musicians playing together" .)