Quiet tune for two (4)
I believe the answer is:
pair
'two' is the definition.
(pair is a kind of two)
'quiet tune' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'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 "Two of a kind, as in cards" , "Cereal used as food" , "Couple - wed" , "Put together - two of a kind" , "item" .)