Think of a number and write it down (7)

I believe the answer is:

compose

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'down' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for compose that I've seen before include "What Ireland used to do" , "Some cop (anag)" , "Write a piece of music" , "Make up, create" , "Create or put together" .)

I've seen this clue in the Evening Standard.

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