Bear disaster at Birkenhead (5)
I believe the answer is:
bruin
'bear' is the definition.
(term for a bear in children's stories)
'disaster at birkenhead' is the wordplay.
'disaster' becomes 'ruin' (ruin can mean disaster or catastrophe).
'at' means one lot of letters go next to another (I've seen this in other clues).
'birkenhead' becomes 'b' (head letter of 'birken').
'ruin' after 'b' is 'BRUIN'.
(Other definitions for bruin that I've seen before include "Archetypal name for a bear" , "Conventional name for a bear" .)