Plenty in favour of merger (9)
I believe the answer is:
profusion
'plenty' is the definition.
(I've seen this before)
'in favour of merger' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'merger' becomes 'fusion' (synonyms).
'pro'+'fusion'='PROFUSION'
(Other definitions for profusion that I've seen before include "Plenty" , "Large number" , "Lots" , "Abundance" .)