Crossword Genius

Sustenance during alimentary abstinence? (4,4)

Ross

I believe the answer is:

fast food

I'm a little stuck... Click here to teach me more about this clue!

'sustenance' is the definition.
(thesaurus)

'during alimentary abstinence?' is the wordplay.
I cannot quite understand how this works, but
'abstinence?' could be 'fast' (fast is a kind of abstinence) and 'fast' is found within the answer.
The remaining letters 'food' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for fast food that I've seen before include "Fish and chips, burgers, etc" , "Burgers or kebabs (but not snails!)" .)

I've seen this clue in The Hindu.
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