Most head off in mature reply (7)
I believe the answer is:
riposte
'reply' is the definition.
(riposting is a kind of replying)
'most head off in mature' is the wordplay.
'head off' means to remove the first letter.
'in' is an insertion indicator.
'mature' becomes 'ripe' (similar in meaning).
'most' with its first letter removed is 'ost'.
'ost' inserted inside 'ripe' is 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Counter-thrust" , "Witty comeback" , "Quick retort or return hit in fencing" , "Witty retort" , "Reply, retort" .)