Handbook describes part of organ? (6)

I believe the answer is:

manual

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'handbook' is the definition.
(manual is a kind of handbook)

I don't understand the rest of the clue.

(Other definitions for manual that I've seen before include "Done by hand, or a handbook" , "Handy" , "Involving human effort" , "Physical hard work" , "Work-book" .)

I've seen this clue in The Irish Times.

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