Good to take on threatening shape in darkness (5)
I believe the answer is:
gloom
'darkness' is the definition.
(gloom is a kind of darkness)
'good to take on threatening shape' is the wordplay.
'good' becomes 'g' (abbreviation).
'to take on threatening shape' becomes 'loom' (to loom over someone is to stand threateningly).
'g'+'loom'='GLOOM'
'in' is the link.
(Other definitions for gloom that I've seen before include "Depression and melancholy" , "Murkiness" , "Despondency" , "Partial darkness" , "Darkness; depression" .)