Feud's conclusion welcomed by ex-serviceman with cheers (8)
I believe the answer is:
vendetta
'feud's' is the definition.
(vendetta is a kind of feud)
'conclusion welcomed by ex-serviceman with cheers' is the wordplay.
'conclusion' becomes 'end' (end is a kind of conclusion).
'welcomed by' is an insertion indicator (I've seen this in other clues).
'ex-serviceman' becomes 'vet' ('vet' can be a synonym of 'ex-serviceman').
'with' is a charade indicator (letters next to each other).
'cheers' becomes 'ta' (I've seen this in other clues).
'end' put inside 'vet' is 'vendet'.
'vendet'+'ta'='VENDETTA'
(Other definitions for vendetta that I've seen before include "ongoing row" , "Family blood feud" , "murderous campaign?" , "Violent quarrel" , "Bitter feud between families" .)