Letter I posted including reply (7)
I believe the answer is:
riposte
'reply' is the definition.
(riposte is a kind of reply)
'letter i posted including' is the wordplay.
'including' says the answer is hidden in the clue.
'RIPOSTE' is hidden in the letters of 'letter i posted'.
(Other definitions for riposte that I've seen before include "Return thrust or repartee" , "Quick retort or return hit in fencing" , "Quick and witty reply" , "Poster I alter for repartee" , "Witty retort" .)