Share son's piano books (5)
I believe the answer is:
'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).
(Other definitions for split that I've seen before include "Mediterranean port" , "shop" , "Rent" , "Cleave or divide" , "Divided" .)