Fine following morning on the rampage (4)
I believe the answer is:
amok
'on the rampage' is the definition.
(I've seen this in another clue)
'fine following morning' is the wordplay.
'fine' becomes 'ok' (synonyms).
'following' says to put letters next to each other.
'morning' becomes 'am' (as in 12:00 AM).
'ok' put after 'am' is 'AMOK'.
(Other definitions for amok that I've seen before include "Going mad" , "Must run it" , "Frenzied, running out of control" , "Frenziedly" , "Berserk" .)