Bear disaster after a bad start (5)
I believe the answer is:
bruin
'bear' is the definition.
(term for a bear in children's stories)
'disaster after a bad start' is the wordplay.
'disaster' becomes 'ruin' (ruin can mean disaster or catastrophe).
'after' says to put letters next to each other.
'start' says to take the initial letters.
The first letter of 'bad' is 'b'.
'ruin' put after 'b' is 'BRUIN'.
(Other definitions for bruin that I've seen before include "Archetypal name for a bear" , "Conventional name for a bear" .)