Ringing old relative, interrupting sleep (8)
I believe the answer is:
resonant
'ringing' is the definition.
(I've seen this before)
'old relative interrupting sleep' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'relative' becomes 'nan' (informal term for a grandmother).
'interrupting' indicates putting letters inside.
'sleep' becomes 'rest' (synonyms).
'o'+'nan'='onan'
'onan' going into 'rest' is 'RESONANT'.
(Other definitions for resonant that I've seen before include "Deep and vibrating" , "Tending to prolong sound, echoing" , "Reverberating" , "Ringing - earn tons (anag)" , "Like" .)