Old soldier cheers, about to finish ongoing conflict (8)
I believe the answer is:
vendetta
'ongoing conflict' is the definition.
(a vendetta is an ongoing feud or conflict)
'old soldier cheers about to finish' is the wordplay.
I cannot really understand how this works, but
'cheers' could be 'ta' (both can informally mean thank you) and 'ta' is found within the answer.
'finish' could be 'end' (synonyms) and 'end' is located in the answer.
'soldier' could be 'vet' (I have seen 'old soldier' mean 'vet' so perhaps 'soldier' could also mean 'vet') and 'vet' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for vendetta that I've seen before include "A bitter Italian feud" , "rivalry" , "A prolonged feud, associated with Sicily" , "intense feud" , "murderous campaign?" .)