Dreadful rage after the first of December (4)
I believe the answer is:
dire
'dreadful' is the definition.
(I know that dreadful can be written as dire)
'rage after the first of december' is the wordplay.
'rage' becomes 'ire' (ire is intense rage).
'after' means one lot of letters go next to another.
'the first of december' becomes 'd' (1st letter of 'december').
'ire' put after 'd' is 'DIRE'.
(Other definitions for dire that I've seen before include "Fearful" , "Desperate; dreadful" , "Of very poor quality" , "Portentous" , "Ride off in dreadful way" .)