Back of a 45 record (1,4)
I believe the answer is:
b side
'back of a 45 record' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for b side that I've seen before include "Pop single's second song" , "Single's less good song" , "most singles had one" , "Less important number of a pop single" .)