Pour in dry white wine, we're told, as and when (2,3)

I believe the answer is:

ad hoc

'as and when' is the definition.
(ad hoc means whenever necessary)

I can't explain the rest of the clue.

(Other definitions for ad hoc that I've seen before include "For one particular purpose" , "For one specific case" , "as required" , "special operation" , "... without preparation" .)

I've seen this clue in The Guardian.

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