For a start, it may be doubled in bookkeeping (5)

I believe the answer is:

entry

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'for a start' is the definition.
(entry is a kind of start)

I don't understand how the remainder of the clue works.

(Other definitions for entry that I've seen before include "field" , "List of candidates" , "Record - number of contestants" , "Doorway" , "The act of going in" .)

I've seen this clue in the Evening Standard.

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