Gradually decreasing sound at end of recording (8)
I believe the answer is:
tapering
'gradually decreasing' is the definition.
(to taper is to get smaller or diminish)
'sound at end of recording' is the wordplay.
'sound' becomes 'ring' (ringing is a kind of sounding).
'at end of' means one lot of letters go next to another (some letters go at the end of others).
'recording' becomes 'tape' (tape recording is a kind of recording).
'ring' put after 'tape' is 'TAPERING'.
(Other definitions for tapering that I've seen before include "Becoming gradually narrower like some candles (8)" , "Narrowing to a point" , "thinning out" , "Becoming less" , "getting less and less" .)