Singularly old-fashioned sound reproduction (4)
I believe the answer is:
mono
'singularly old-fashioned sound reproduction' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'singularly old' is the wordplay.
'singularly' becomes 'mon' (I can't explain this - if you can you should give a lot more credence to this answer).
'old' becomes 'o' (abbreviation).
'mon'+'o'='MONO'
Can you help me to learn more?
(Other definitions for mono that I've seen before include "Single-speaker sound" , "Not stereo" , "Single-channel sound" , "Transmitting over one channel" .)