One retaliating against egghead in irritation (7)
I believe the answer is:
avenger
'one retaliating' is the definition.
(I've seen this before)
'against egghead in irritation' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'egghead' becomes 'e' (head letter of 'egg').
'in' means one lot of letters goes inside another.
'irritation' becomes 'anger'.
'v'+'e'='ve'
've' placed into 'anger' is 'AVENGER'.
(Other definitions for avenger that I've seen before include "party retaliating" , "one evening?" , "Bringer of retribution" , "One settles score" , "Someone seeking to settle a score" .)