Disaster, taking on black bear (5)
I believe the answer is:
bruin
'bear' is the definition.
(term for a bear in children's stories)
'disaster taking on black' is the wordplay.
'disaster' becomes 'ruin' (ruin can mean disaster or catastrophe).
'taking on' says to put letters next to each other.
'black' becomes 'b' (as in pencil classifications).
'ruin' put after 'b' is 'BRUIN'.
(Other definitions for bruin that I've seen before include "Conventional name for a bear" , "Archetypal name for a bear" .)