Fast food (4)
I believe this is a double definition.
'fast' is the first definition.
(diet is a kind of fast)
'food' is the second definition.
(diet is a kind of food)
(Other definitions for diet that I've seen before include "Regime followed by slimmer" , "(Perhaps, try to) reduce rump; parliament" , "Restriction of food intake" , "Slimmer's regimen" , "provisions" .)