Stretches a winning margin with ensign (9)

I believe the answer is:

lengthens

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'stretches' is the definition.
The definition and answer can be both related to change as well as being verbs in their -s form.
Perhaps you can see an association between them that I can't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for lengthens that I've seen is " extends".)

I've seen this clue in the Sydney Morning Herald.

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