EU actions involved firm (9)
I believe the answer is:
tenacious
'firm' is the definition.
(thesaurus)
'eu actions involved' is the wordplay.
'involved' is an anagram indicator (involved can mean confusing or complex).
'eu'+'actions'='euactions'
'euactions' is an anagram of 'TENACIOUS'.
(Other definitions for tenacious that I've seen before include "Persevering" , "Keeping a firm hold" , "Holding fast" , "Dogged" , "Determined" .)