Astronomical work, in essence an old tale of adventure (8)

I believe the answer is:

almagest

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'astronomical work' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for almagest that I've seen before include "Astronomical work" , "description of stars?" .)

I've seen this clue in The Sunday Times and The Times.

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