EU right to upset firm (4)
I believe the answer is:
true
'firm' is the definition.
(thesaurus)
'eu right to upset' is the wordplay.
'right' becomes 'rt'.
'to upset' says the letters should be written in reverse.
'eu'+'rt'='eurt'
'eurt' backwards is 'TRUE'.
(Other definitions for true that I've seen before include "Veracious" , "A false alternative?" , "In accordance with facts" , "Honest and straight" , "Right - honourable" .)