I'd flipped over, repose being most terrible (6)
I believe the answer is:
direst
'most terrible' is the definition.
('dire' can be a synonym of 'terrible')
'i'd flipped over repose' is the wordplay.
'flipped over' shows that the letters should be reversed in order.
'repose' becomes 'rest' ('rest' can be a synonym of 'repose').
'id' back-to-front is 'di'.
'di'+'rest'='DIREST'
'being' is the link.
(Other definitions for direst that I've seen before include "far from ideal" , "Most horrific" .)