Brexit negotiator swaps note with Republican fence (7)
I believe the answer is:
barrier
'fence' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for barrier that I've seen before include "Obstacle to movement" , "Obstacle to progress" , "it's soundly broken!" , "See 4" , "Structure preventing access" .)