Terrible conclusion of road rage (4)
I believe the answer is:
dire
'terrible' is the definition.
(I know that terrible can be written as dire)
'conclusion of road rage' is the wordplay.
'conclusion of' says to take the final letters.
'rage' becomes 'ire' (I've seen this before).
The last letter of 'road' is 'd'.
'd'+'ire'='DIRE'
(Other definitions for dire that I've seen before include "Terrible display" , "Really awful kind of ride" , "Ride off in dreadful way" , "Desperate; dreadful" , "Extremely serious or urgent" .)