Guarantee to knock out right result (5)
I believe the answer is:
ensue
'result' is the definition.
('ensue' can be a synonym of 'result')
'guarantee to knock out right' is the wordplay.
'guarantee' becomes 'ensure' ('ensure' can be a synonym of 'guarantee').
'to knock out' indicates named letters should be taken away.
'right' becomes 'r' (common abbreviation).
'ensure' with 'r' taken out is 'ENSUE'.
(Other definitions for ensue that I've seen before include "Transpire" , "Happen - to come later" , "Come next" , "Come along afterwards" , "Come about, result in" .)