She doesn’t believe in fair diet, regularly draining litre (7)

I believe the answer is:

infidel

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'she doesn't believe' is the definition.
I can't tell whether this definition defines the answer.

'in fair diet regularly draining litre' is the wordplay.
'regularly draining' indicates alternate letters (I've seen 'regularly drained' mean this).
'litre' becomes 'L' (abbreviation).
'fair'+'diet'='fairdiet'
The alternate letters of 'fairdiet' are 'fide'.
'in'+'fide'+'l'='INFIDEL'

Can you help me to learn more?

(Other definitions for infidel that I've seen before include "Person who rejects a religious belief" , "Disbeliever" , "Person not operating in good faith" , "Someone who does not acknowledge your god" , "One who doesn't believe" .)

I've seen this clue in the Sydney Morning Herald.

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