Main problem for the Brexiteers (5,3)

I believe the answer is:

irish sea

I'm a little stuck... Click here to teach me more about this clue!

'main' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for irish sea that I've seen before include "It separates Ireland from Britain" , "Man's situation" , "Main" , "Waters separating Ireland from England and Wales" , "It separates Ireland from England and Wales" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.