Not up for finishing off sauce boat (5)

I believe the answer is:

ketch

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'boat' is the definition.
(ketch is a type of sailing boat)

I cannot understand how the rest of the clue works.

(Other definitions for ketch that I've seen before include "2-masted boat" , "Small sailing vessel with two masts" , "Two-masted sailing ship" , "The craft" , "Two-masted sailing vessel" .)

I've seen this clue in The Times.

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