Tiger, say, having a fit of bad temper -- he's a disaster! (11)
I believe the answer is:
catastrophe
'a disaster' is the definition.
('catastrophe' can be a synonym of 'disaster')
'tiger say having a fit of bad temper he's' is the wordplay.
'tiger say' becomes 'cat' (tiger is a kind of cat).
'having' means one lot of letters go next to another.
'fit of bad temper' becomes 'strop' (I've seen this before).
'cat'+'a'+'strop'+'he'='CATASTROPHE'
(Other definitions for catastrophe that I've seen before include "Denouement" , "final event" , "disastrous event?" , "Great disaster" , "Utter disaster" .)