Join forces with idiot stopping rise of European Union (5)
I believe the answer is:
unite
'join forces' is the definition.
(thesaurus)
'idiot stopping rise of european union' is the wordplay.
'idiot' becomes 'nit' (I've seen this before).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'rise of' says the letters should be written in reverse.
'european union' becomes 'eu'.
'eu' written backwards gives 'ue'.
'nit' inserted into 'ue' is 'UNITE'.
'with' is the link.
(Other definitions for unite that I've seen before include "Become an integrated whole" , "Club together" , "Wed?" , "Come, or join together" , "Join together, become one" .)