Being unconscious after a short right leads to complete defeat (4)
I believe the answer is:
rout
'complete defeat' is the definition.
(I've seen this before)
'being unconscious after a short right' is the wordplay.
'being unconscious' becomes 'out' (similar in meaning).
'after' means one lot of letters go next to another.
'a short right' becomes 'r' (abbreviation; 'short' suggests a short form).
'out' put after 'r' is 'ROUT'.
'leads to' acts as a link.
(Other definitions for rout that I've seen before include "Inflict heavy defeat on" , "Defeat thoroughly" , "Decisive defeat" , "Defeat comprehensively" , "Trounce" .)