Bookcase part (5)
I believe the answer is:
shelf
'bookcase part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for shelf that I've seen before include "Ledge for books etc" , "Storage ledge" , "Ledge for holding things" , "Whereon 2 down is not!" , "Horizontal surface for holding things" .)