Diana's bloomer is a catastrophe (8)
I believe the answer is:
disaster
'a catastrophe' is the definition.
('disaster' can be a synonym of 'catastrophe')
'diana's bloomer' is the wordplay.
'diana's' becomes 'Di's' (Di is short for Diana).
'bloomer' becomes 'aster' (aster is a kind of flowering plant).
'dis'+'aster'='DISASTER'
'is' is the link.
(Other definitions for disaster that I've seen before include "Star dies (anag)" , "Extreme event resulting in great loss and misfortune" , "Sudden catastrophe" , "complete failure" , "Event causing great damage" .)