Blacken or scorch outside, medium inside (5)

I believe the answer is:

smear

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'blacken' is the definition.
(I've seen this in another clue)

'scorch outside medium inside' is the wordplay.
'scorch' becomes 'sear' (synonyms).
'outside medium' becomes 'm' (as in UK clothing sizes. I am not sure about the 'outside' bit.).
'inside' indicates putting letters inside.
'sear' going around 'm' is 'SMEAR'.

'or' is the link.

Can you help me to learn more?

(Other definitions for smear that I've seen before include "Slanderous denigration of character" , "Smother" , "material for examination that is on the slide" , "Dirty mark; aspersion" , "Greasy/sticky mark" .)

I've seen this clue in The Times.

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