Misunderstanding caused by argument against merger (9)
I believe the answer is:
confusion
'misunderstanding' is the definition.
(thesaurus)
'argument against merger' is the wordplay.
'argument' becomes 'con' (con is a kind of argument**).
'against' is a charade indicator (letters next to each other).
'merger' becomes 'fusion' ('fusion' can be a synonym of 'merger').
'con'+'fusion'='CONFUSION'
'caused by' acts as a link.
(Other definitions for confusion that I've seen before include "Uncertainty" , "Chaos" , "Perplexity" , "Shame!" , "Discombobulation, disarray" .)