Hit back at volunteers employed by online seller making a comeback (9)
I believe the answer is:
retaliate
'hit back' is the definition.
(I've seen this before)
'volunteers employed by online seller making a comeback' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'employed by' indicates putting letters inside (inserted letters used or employed inside).
'online seller' becomes 'e tailer' (online retailer).
'making a comeback' shows that the letters should be reversed in order.
'etailer' reversed gives 'reliate'.
'ta' inserted into 'reliate' is 'RETALIATE'.
'at' acts as a link.
(Other definitions for retaliate that I've seen before include "settle accounts" , "Make tit-for-tat attack" , "Fight back" , "Get revenge" , "Exact retribution" .)