Blemish making Spooner cry with laughter? (9)

I believe the answer is:

birthmark

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

'blemish' is the definition.
(birthmark is a kind of blemish)

I don't understand the rest of the clue.

(Other definitions for birthmark that I've seen before include "Naevus" , "Blemish on the skin" , "Skin pigmentation" .)

I've seen this clue in The Sun.

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