One's witty greeting in post (4)

I believe the answer is:

card

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

'greeting in post' is the definition.
'card' can be an answer for 'greeting' (card is a kind of greeting). I'm not certain of the 'in post' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for card that I've seen before include "Bit of a wag - joker, perhaps" , "Posted greeting" , "showing heart, say" , "Jack, for one" , "What may be used for greeting" .)

I've seen this clue in The Sun.

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