Letter I posted contains reply (7)
I believe the answer is:
riposte
'reply' is the definition.
(riposte is a kind of reply)
'letter i posted contains' is the wordplay.
'contains' indicates a hidden word.
'RIPOSTE' is hidden within 'letter i posted'.
(Other definitions for riposte that I've seen before include "Repartee; quick thrust (fencing)" , "Rejoin" , "Smart reply" , "Reply, response" , "Quick retort or return thrust in fencing" .)