Cancel Britain's terminal European exit (6)
I believe the answer is:
negate
'cancel' is the definition.
(I've seen this before)
'britain's terminal european exit' is the wordplay.
'terminal' suggests the final letters (the final or terminal letter).
'european' becomes 'e' (abbreviation e.g. EU).
'exit' becomes 'gate' (I've seen this before).
The final letter of 'britain' is 'n'.
'n'+'e'+'gate'='NEGATE'
(Other definitions for negate that I've seen before include "Disprove or nullify" , "Show to be false" , "Cancel; invalidate" , "Nullify - deny the existence of" , "Opera by Puccini" .)