Fast food (4)
This clue is a double definition.
'fast' is the first definition.
(I know that diet is a type 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" .)
'fast food' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cheeseburger that I've seen before include "filled roll" , "Fast food item" , "Fast food product" , "Fast food -- begs cure here (anag)" , "Fast food snack" .)