Small spot on ermine (5)

I believe the answer is:

stoat

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'ermine' is the definition.
(I know that ermine can be written as Stoat)

'small spot on' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'spot' becomes 'to' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' becomes 'at' (I've seen this in other clues).
's'+'to'+'at'='STOAT'

Can you help me to learn more?

(Other definitions for stoat that I've seen before include "furry animal" , "Weasel, ermine" , "Short-tail weasel" , "Weasel-like mammal" , "Member of the weasel family" .)

I've seen this clue in The Times.

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