Oppose United and, later, European entering formal agreement (10)
I believe the answer is:
counteract
'and later european entering formal agreement' is the definition.
I can't tell whether this definition defines the answer.
'oppose united' is the wordplay.
'oppose' becomes 'counter' (I've seen this before).
'united' becomes 'act' (I can't explain this - if you can you should believe this answer much more).
'counter'+'act'='COUNTERACT'
(Other definitions for counteract that I've seen before include "Tell against (something)" , "Put a check on" , "Oppose" , "Take steps to neutralise" , "Nullify" .)