Consequence of over-indulgence that could appear in threatening form (3,7,5)
I believe the answer is:
the morning after
'consequence of over-indulgence' is the definition.
(eg the morning after heavy drinking)
'that could appear in threatening form' is the wordplay.
'that could appear in' indicates anagramming the letters.
'threatening'+'form'='threateningform'
'threateningform' is an anagram of 'THE MORNING AFTER'.
(Other definitions for the morning after that I've seen before include "Unpleasant after-effects of an evening's drinking" , "hangover" , "unpleasant consequences" , "Consequences of indulgence" .)