Old Stone in mellow comeback (7)
I believe the answer is:
riposte
'comeback' is the definition.
('riposte' can be a synonym of 'comeback')
'old stone in mellow' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'stone' becomes 'st'.
'in' indicates putting letters inside.
'mellow' becomes 'ripe' ('ripe' can be similar in meaning to 'mellow').
'o'+'st'='ost'
'ost' going within 'ripe' is 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Witty retort" , "Reply, retort" , "Rejoin" , "Sharp answer" , "Fencing counterattack" .)