Not stereo; but on its day, loved (4)
I believe the answer is:
mono
'not stereo' is the definition.
(I've seen this in another clue)
'on its day loved' is the wordplay.
I cannot really see how this works, but
'on' is found in the answer.
The remaining letters 'mo' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'but' acts as a link.
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" .)