Extravagance for joining union (9)
I believe the answer is:
profusion
'extravagance' is the definition.
(thesaurus)
'for joining union' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'joining' says to put letters next to each other.
'union' becomes 'fusion' (fusion is a kind of union**).
'pro'+'fusion'='PROFUSION'
(Other definitions for profusion that I've seen before include "Abundance" , "Plenty" , "Large number" , "Lots" .)