Not stereo; but on its day, loved (4)
I believe the answer is:
mono
'not stereo' is the definition.
(I've seen this before)
'day loved' is the wordplay.
'day' becomes 'mon'.
'loved' becomes 'o' (I am not sure about this - if you are sure you should believe this answer much more).
'mon'+'o'='MONO'
'but on its' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for mono that I've seen before include "Single-channel sound" , "Not stereo" , "Single-speaker sound" , "Transmitting over one channel" .)