Swift sharp reply in letter I posted (7)

I believe the answer is:

riposte

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'swift sharp reply' is the definition.
'riposte' can be an answer for 'reply' (riposte is a kind of reply). I am unsure of the 'swift sharp' bit.

'in letter i posted' is the wordplay.
'in' says the answer is hidden in the clue.
'RIPOSTE' can be found hidden inside 'letter i posted'.

Can you help me to learn more?

(Other definitions for riposte that I've seen before include "Reply, retort" , "Quick retort or return thrust in fencing" , "Fencing counterattack" , "Quick clever reply" , "Quick answer" .)

I've seen this clue in The Telegraph.

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