A mocking letter to a relative (5)

I believe the answer is:

taunt

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'a mocking' is the definition.
I can't judge whether this defines the answer.

'letter to a relative' is the wordplay.
'letter' becomes 't' ().
'to a relative' becomes 'aunt' (aunt is a type of relative).
't'+'aunt'='TAUNT'

Can you help me to learn more?

(Other definitions for taunt that I've seen before include "Mock provokingly" , "Bait with insulting remarks" , "Sarcastic challenge" , "Provoke; tease" , "Rag, goad" .)

I've seen this clue in the Evening Standard.

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