Dreadful rough ride (4)
I believe the answer is:
dire
'dreadful' is the definition.
('dire' can be a synonym of 'dreadful')
'rough ride' is the wordplay.
'rough' indicates an anagram.
'ride' is an anagram of 'DIRE'.
(Other definitions for dire that I've seen before include "Ominous; very bad" , "Cataclysmic" , "Fearful" , "Dreadful; grim" , "Terrible display" .)