Most awful din curtailed repose (6)
I believe the answer is:
direst
'most awful' is the definition.
('dire' can be a synonym of 'awful')
'din curtailed repose' is the wordplay.
'curtailed' means to remove the last letter.
'repose' becomes 'rest' ('rest' can be a synonym of 'repose').
'din' with its last letter taken away is 'di'.
'di'+'rest'='DIREST'
(Other definitions for direst that I've seen before include "far from ideal" , "Most horrific" .)