Surplus footwear? (8)

I believe the answer is:

overshoe

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

'surplus footwear?' is the definition.
'overshoe' can be an answer for 'footwear?' (overshoe is a kind of footwear). I'm unsure of the 'surplus' bit.

'surplus footwear?' is the wordplay.
'surplus' becomes 'over' (I've seen this before).
'footwear?' becomes 'shoe' (shoe is a kind of footwear).
'over'+'shoe'='OVERSHOE'

Can you help me to learn more?

(Other definitions for overshoe that I've seen before include "Re hooves? (anag)" , "Protective item of footwear" , "Galosh" , "one covering Oxford?" , "which may be outside Oxford" .)

I've seen this clue in The Globe and Mail.

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