Loyal Brussels right back (4)
I believe the answer is:
true
'loyal' is the definition.
(I've seen this in another clue)
'brussels right back' is the wordplay.
'brussels' becomes 'EU' (European Parliament is based in Brussels).
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'back' is a reversal indicator.
'eu'+'rt'='eurt'
'eurt' in reverse letter order is 'TRUE'.
(Other definitions for true that I've seen before include "Honest - expression of agreement" , "Factually accurate" , "Genuine, real" , "Exact - loyal - honestly" , "Exact; faithful" .)