Threaten weaver? (4)
I believe the answer is:
loom
This clue is a double definition.
'threaten' is the first definition.
(looming is a kind of threatening)
'weaver?' is the second definition.
(thesaurus)
(Other definitions for loom that I've seen before include "'Appear as a vague form, perhaps threatening (4)'" , "Weaving machine to appear indistinctly" , "weaver" , "Appear indistinctly, in threatening form" , "Menace - weaving device" .)