Appear ominously (4)
I believe the answer is:
loom
'appear ominously' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for loom that I've seen before include "Weaving instrument" , "Weaving machine to appear indistinctly" , "tower" , "Textile machine for weaving" , "Machine for weaving yarn" .)