Threatening, bad guy crossing line (4)
I believe the answer is:
ugly
'threatening' is the definition.
(I've seen this before)
'bad guy crossing line' is the wordplay.
'bad' is an anagram indicator.
'crossing' is an insertion indicator.
'line' becomes 'l'.
'guy' is an anagram of 'ugy'.
'ugy' enclosing 'l' is 'UGLY'.
(Other definitions for ugly that I've seen before include "No oil painting!" , "Difficult to look at" , "Deficient in beauty" , "Threatening; nasty" , "Ill-natured" .)