Unconsciousness of something missing in a fight? (8)
I believe the answer is:
blackout
'unconsciousness' is the definition.
(blackout is a kind of unconsciousness)
'something missing in a fight?' is the wordplay.
'something missing' becomes 'lack' (something wanting or lacking).
'in' is an insertion indicator.
'a fight?' becomes 'bout' (as in a bout in boxing).
'lack' inserted within 'bout' is 'BLACKOUT'.
'of' acts as a link.
(Other definitions for blackout that I've seen before include "Compulsory darkness during wartime, or temporary loss of memory" , "When power's gone" , "Fainting" , "complete dark" , "Temporary loss of consciousness or electric power" .)