The lid to the piano (3)
I believe the answer is:
top
'the lid' is the definition.
(thesaurus)
'to the piano' is the wordplay.
'the piano' becomes 'p'.
'to'+'p'='TOP'
(Other definitions for top that I've seen before include "Highest point or position" , "Peak; beat" , "Toy; uppermost point" , "Pinnacle" , "Crown" .)