Unknown listeners have aeons! (5)
I believe the answer is:
years
'aeons' is the definition.
The answer and definition can be both related to time as well as being plural nouns.
Maybe you can see an association between them that I can't see?
'unknown listeners' is the wordplay.
'unknown' becomes 'y' (algebra).
'listeners' becomes 'ears' (body parts which listen).
'y'+'ears'='YEARS'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for years that I've seen before include "Long periods" , "Units of time" , "Periods of time" , "Ages" , "Very long time (colloq.)" .)