The record's discernibly reduced (4)
I believe the answer is:
disc
'the record's' is the definition.
(eg a CD, compact disc)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for disc that I've seen before include "One of 33, 45 or 78?" , "Old record" , "Spine cartilage layer" , "Back part" , "'Round, flat shape (4)'" .)