Run off to form a union (5)

I believe the answer is:

elope

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'run off' is the definition.
(I know that run off can be written as elope)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for elope that I've seen before include "(Lovers) run away" , "Run away to Gretna Green?" , "leave with lover" , "Run away with one's beloved" , "Run away secretly to marry" .)

I've seen this clue in The Daily Express.

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