Nutrition doctor's writing stuff on low-calorie soda (4,5)

I believe the answer is:

diet drink

'low-calorie soda' is the definition.
(drink which is low in calories)

'nutrition doctor's writing stuff on' is the wordplay.
'nutrition' becomes 'diet' (both can mean food).
'doctor' becomes 'Dr' (abbreviation).
'writing stuff' becomes 'ink' (liquid used for writing).
'on' means one lot of letters go next to another.
'diet'+'dr'+'ink'='DIET DRINK'

I've seen this clue in The Guardian.

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