Dreadful bumpy ride (4)
I believe the answer is:
dire
'dreadful' is the definition.
(I know that dreadful can be written as dire)
'bumpy ride' is the wordplay.
'bumpy' indicates anagramming the letters (letters get bumped around).
'ride' is an anagram of 'DIRE'.
(Other definitions for dire that I've seen before include "Appalling or extreme" , "'Dreadful, awful (4)'" , "Desperate; dreadful" , "Ride around can be calamitous" , "Terrible display" .)