Certain merger stopped by crusade against Dutch bank (13)
I believe the answer is:
unquestioning
'certain' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'merger stopped by crusade against dutch bank' is the wordplay.
'merger' becomes 'union' (I've seen this before).
'stopped by' means one lot of letters goes inside another (stopping can mean blocking a gap).
'crusade' becomes 'quest' (I can't justify this - if you can you should believe this answer much more).
'against' is a charade indicator (letters next to each other).
'dutch bank' becomes 'ING' (ING Group, Dutch banking corporation).
'union' enclosing 'quest' is 'unquestion'.
'unquestion'+'ing'='UNQUESTIONING'
Can you help me to learn more?
(Another definition for unquestioning that I've seen is " Not expressing doubt".)