Brussels upset after Brexiters’ final charge (3)
I believe the answer is:
sue
'charge' is the definition.
(I've seen this before)
'brussels upset after brexiters' final' is the wordplay.
'brussels' becomes 'eu' (Brussels is a metonym for the European Union).
'upset' says the letters should be written backwards.
'after' means one lot of letters go next to another.
'final' says to take the final letters.
The last letter of 'brexiters' is 's'.
'eu' back-to-front is 'ue'.
'ue' put after 's' is 'SUE'.
(Other definitions for sue that I've seen before include "Charge" , "Prosecute at law" , "Take legal case against" , "Little Susan" , "Plead" .)