Unconscious in fight, getting head struck (3)
I believe the answer is:
out
'unconscious' is the definition.
('out' can be similar in meaning to 'unconscious')
'fight getting head struck' is the wordplay.
'fight' becomes 'bout' ('bout' can be a synonym of 'fight').
'getting head struck' means to remove the first letter.
'bout' with its initial letter taken away is 'OUT'.
'in' is the link.
(Other definitions for out that I've seen before include "Defeated" , "Dismissed, in cricket" , "Walking back to the pavilion" , "Means of escape" , "Revealed; unconscious" .)