Endless ring makes a measure of sound (3)
I believe the answer is:
bel
'a measure of sound' is the definition.
(as in decibel)
'endless ring' is the wordplay.
'endless' means to remove the last letter.
'ring' becomes 'bell' (I've seen this before).
'bell' with its last letter taken away is 'BEL'.
'makes' is the link.
(Other definitions for bel that I've seen before include "Unit relating to sound intensity" , "Babylonian god" , "Unit of volume" , "starts apocryphal work" , "Some noise" .)