Piano tune for two (4)
I believe the answer is:
pair
'two' is the definition.
(pair is a kind of two)
'piano tune' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'tune' becomes 'air' (air can mean a piece of music).
'p'+'air'='PAIR'
'for' is the link.
(Other definitions for pair that I've seen before include "flight of stairs" , "Cereal used as food" , "Put together - two of a kind" , "Twins" , "Two ducks in one game of cricket" .)