Share son's piano books (5)
I believe the answer is:
split
'share' is the definition.
(split can mean someone's share of something)
'son's piano books' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'books' becomes 'lit' (abbreviation for literature).
's'+'p'+'lit'='SPLIT'
(Other definitions for split that I've seen before include "Mediterranean port" , "shop" , "Rent" , "Cleave or divide" , "Divided" .)