Rodgers and Hart song: low, low number (4,4)
I believe the answer is:
blue moon
'rodgers and hart song' is the definition.
(Blue Moon is a song by Rodgers and Hart)
'low low number' is the wordplay.
'low' becomes 'blue' (both can mean unhappy).
'low' becomes 'moo' (both can mean to make a cow noise).
'number' becomes 'n' (n can represent any number).
'blue'+'moo'+'n'='BLUE MOON'
(Other definitions for blue moon that I've seen before include "In which once is very seldom" , "If something happens once in it, it's not very often" , "Very rare occurrence" , "Rodgers and Hart song" , "Once in one is a rare event" .)