Ringing old relative to stop sleep (8)
I believe the answer is:
resonant
'ringing' is the definition.
(I've seen this before)
'old relative to stop sleep' is the wordplay.
'old' becomes 'o' (abbreviation).
'relative' becomes 'nan' (informal term for a grandmother).
'to stop' indicates putting letters inside (stopping can mean blocking a gap).
'sleep' becomes 'rest' (synonyms).
'o'+'nan'='onan'
'onan' put inside 'rest' is 'RESONANT'.
(Other definitions for resonant that I've seen before include "Continuing to sound clearly and deeply" , "(Of sound) deep, clear and continuing to ring" , "Deep, clear and continuing to sound or reverberate" , "Resounding, ringing" , "Reverberating" .)