Keep book under piano (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep can be written as preserve)
'book under piano' is the wordplay.
'book' becomes 'reserve' (as in booking or reserving in advance).
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'piano' becomes 'p' (musical abbreviation).
'reserve' put after 'p' is 'PRESERVE'.
(Other definitions for preserve that I've seen before include "Keep safe or in unchanged condition" , "Keep alive; jam" , "Maintain in unaltered condition" , "Protect; jam" , "Eg, jam" .)