WWII belligerent to criticise Munich Agreement at first (5)
I believe the answer is:
japan
'wwii belligerent' is the definition.
(Japan was a belligerent in World War II)
'criticise munich agreement at first' is the wordplay.
'criticise' becomes 'pan' (informal term meaning to criticise heavily).
'munich agreement' becomes 'ja' ('yes' in German).
'at first' means one lot of letters go next to another.
'pan' after 'ja' is 'JAPAN'.
'to' acts as a link.
(Other definitions for japan that I've seen before include "Glossy black lacquer" , "Country in East Asia" , "Twelve [VARNISH]" , "Glossy, black varnish; country" , "Country in Far East" .)