Check about objective before reserves feud (8)
I believe the answer is:
vendetta
'feud' is the definition.
(vendetta is a kind of feud)
'check about objective before reserves' is the wordplay.
'check' becomes 'vet' (I've seen this in other clues**).
'about' is an insertion indicator.
'objective' becomes 'end' (I've seen this before).
'before' says to put letters next to each other.
'reserves' becomes 'ta' (I've seen this in other clues).
'vet' enclosing 'end' is 'vendet'.
'vendet'+'ta'='VENDETTA'
(Other definitions for vendetta that I've seen before include "A bitter Italian feud" , "Blood-feud between families" , "ding-dong battle" , "Violent quarrel" , "Murderous feud" .)