The elderly quietly agreed (3)
I believe the answer is:
yep
'agreed' is the definition.
(both are expressions of agreement)
'the elderly quietly' is the wordplay.
'the elderly' becomes 'ye' ('ye' resembles an archaic spelling of 'the').
'quietly' becomes 'p' (p for piano and quiet).
'ye'+'p'='YEP'
(Other definitions for yep that I've seen before include "Correct" , "Affirmative (US colloq.)" , "Positive reaction" , "informal approval" .)