Bear black disaster (5)
I believe the answer is:
bruin
'bear' is the definition.
(bruin is a kind of bear)
'black disaster' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'disaster' becomes 'ruin' (ruin can mean disaster or catastrophe).
'b'+'ruin'='BRUIN'
(Other definitions for bruin that I've seen before include "Conventional name for a bear" , "Archetypal name for a bear" .)