Fast food (4)

I think it could be either:

Which length description are you looking for?:

7,6

diet

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" .)

I've seen this clue in The Guardian.

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