Catastrophe as diet's ruined with little resistance (8)
I believe the answer is:
disaster
'catastrophe' is the definition.
(I know that catastrophe can be written as disaster)
'as diet's ruined with little resistance' is the wordplay.
'ruined with' indicates an anagram.
'little' indicates taking the first letters.
The initial letter of 'resistance' is 'r'.
'as'+'diets'='asdiets'
'asdiets' with its letters mixed up with 'r' gives 'DISASTER'.
(Other definitions for disaster that I've seen before include "Event causing great damage" , "Star dies (anag)" , "terrible trouble" , "Extreme ruin or misfortune" , "Catastrophe, calamity" .)