Join European after treating problem (9)
I believe the answer is:
integrate
'join' is the definition.
(I've seen this before)
'european after treating problem' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'after' means one lot of letters go next to another.
'problem' is an anagram indicator.
'treating' is an anagram of 'integrat'.
'e' after 'integrat' is 'INTEGRATE'.
(Other definitions for integrate that I've seen before include "Mix" , "Incorporate into the main group" , "Get together" , "Join" , "unify" .)