Answer extract from letter I posted (7)
I believe the answer is:
riposte
'answer' is the definition.
(I know that riposte is a type of answer)
'extract from letter i posted' is the wordplay.
'extract from' indicates the answer is hidden within the clue (some letters extracted from the clue).
'RIPOSTE' is hidden in the letters of 'letter i posted'.
(Other definitions for riposte that I've seen before include "Witty comeback" , "Sally" , "Return thrust or repartee" , "Reply, response" , "Quick answer" .)