Unwanted marks, we hear, on the map of Surrey (7)

I believe the answer is:

staines

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'unwanted marks' is the definition.
The answer and definition can be both related to change as well as being verbs in their -s form.
Perhaps there's a link between them I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for staines that I've seen before include "Surrey town on Thames" , "Town SW of Heathrow" .)

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