Complete bill of lading form. Don't mention old bag (4,2)

I believe the answer is:

fill in

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'complete' is the definition.
(as in completing a questionnaire)

I don't understand the remainder of the clue.

(Other definitions for fill in that I've seen before include "'Act as temporary substitute (4,2)'" , "Brief" , "Occupy (time)" , "Deputise - complete form" , "point between courses" .)

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

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