Most unpleasant defeat (5)
I believe the answer is:
worst
I believe 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)
(Other definitions for worst that I've seen before include "Least competent" , "Least well" , "Most inferior or evil" , "Far from the best - defeat" , "The least favorable" .)