Crossword Genius

Unfinished book shelf (5)

Ross

I believe the answer is:

ledge

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'unfinished book' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'shelf' is the second definition.
(I know that shelf can be written as ledge)

Can you help me to learn more?

(Other definitions for ledge that I've seen before include "Surface under e.g. window" , "Ability to contain one's emotions" , "Narrow projecting shelf as from window" , "rocky situation?" , "Mantelpiece" .)

I've seen this clue in The Globe and Mail.
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