Word ending a threat (4)
I believe the answer is:
else
'word ending a threat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for else that I've seen before include "Other than (what is under consideration)" , "Besides, otherwise" , "As an alternative" , "Otherwise; different from" , "Adverb meaning if not, otherwise" .)