Alliance if led by British would be a cause of pain (5)

I believe the answer is:

union

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

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

I don't understand how the remainder of the clue works.

(Other definitions for union that I've seen before include "workers' co-operative" , "Association with a common purpose" , "State of being joined together" , "The joining of two or more together" , "See 22" .)

I've seen this clue in The Times.

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.