Epitaph, nothing - set out a reply (7)
I believe the answer is:
riposte
'a reply' is the definition.
(I know that riposte is a more specific form of the action answer)
'epitaph nothing set out' is the wordplay.
'epitaph' becomes 'RIP' (rest in peace).
'nothing' becomes 'O' (looks like zero - 0).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'set' with letters rearranged gives 'ste'.
'rip'+'o'+'ste'='RIPOSTE'
(Other definitions for riposte that I've seen before include "Snappy comeback" , "Witty comeback" , "Fencing counterattack" , "Sharp answer" , "Quick answer" .)