Keep book under piano (8)
I believe the answer is:
preserve
'keep' is the definition.
(I know that keep is a more specific form of enter)
'book under piano' is the wordplay.
'book' becomes 're-serve' (as in booking or reserving in advance).
'under' means one lot of letters go next to another (below, in a down clue).
'piano' becomes 'p' (musical abbreviation).
'reserve' 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" .)