Crossword Genius

Paid in post but badly let down (10)

Ross

I believe the answer is:

disappoint

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

'let down' is the definition.
(I know that let down can be written as disappoint)

'paid in post but badly' is the wordplay.
'paid in post' anagrams to 'DISAPPOINT'.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for disappoint that I've seen before include "Fail to satisfy" , "Let down" .)

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