Most awful defeat (5)
I believe the answer is:
worst
I believe this clue is a double definition.
'most awful' is the first definition.
(I've seen this in another clue)
'defeat' is the second definition.
(worst can mean to defeat)
(Other definitions for worst that I've seen before include "The least favorable" , "Lowest of the low" , "Most inferior or evil" , "the pits" , "The least favourable" .)