What English, Scots, Welsh & Loyalists have in common? (5,4)

I believe the answer is:

union flag

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

'common?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for union flag that I've seen before include "Jack" , "national standards collectively" .)

I've seen this clue in The Guardian.

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