Surplus pennies pocketed by Brexit opponent (9)

I believe the answer is:

remainder

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

'surplus' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for remainder that I've seen before include "Surplus" , "Leftovers" , "deposit" , "set of unsold books" , "Left-over" .)

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

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