Having had a fight backs out of the engagement (8)

I believe the answer is:

retreats

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'backs out of the engagement' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for retreats that I've seen before include "tactical withdrawals" , "Goes back, like defeated army" , "Refuges" , "Places that afford peace and quiet" , "Recedes" .)

I've seen this clue in the Evening Standard.

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