Agent backing boy band together and individually (10)
I believe the answer is:
personally
'individually' is the definition.
(thesaurus)
'agent backing boy band together' is the wordplay.
'agent' becomes 'rep' (short for representative).
'backing' shows that the letters should be reversed in order.
'boy' becomes 'son' (synonyms).
'band together' becomes 'ally' (I've seen this before).
'rep' in reverse letter order is 'per'.
'per'+'son'+'ally'='PERSONALLY'
'and' is the link.
(Other definitions for personally that I've seen before include "Speaking for myself" , "in my opinion" , "I think" , "in my book" , "as far as one is concerned" .)