Most unpleasant in defeat (5)
I believe the answer is:
worst
This clue is a double definition.
'most unpleasant' is the first definition.
('worst' can be similar in meaning to 'most unpleasant')
'defeat' is the second definition.
(worsting is a kind of defeating)
'in' is the link.
(Other definitions for worst that I've seen before include "Of the poorest quality" , "Direst" , "Beat" , "Most unpleasant" , "Least competent" .)