Unknown listeners for a long time (5)
I believe the answer is:
years
'long time' is the definition.
(synonyms)
'unknown listeners' is the wordplay.
'unknown' becomes 'y' (algebra).
'listeners' becomes 'ears' (body parts which listen).
'y'+'ears'='YEARS'
'for a' is the link.
(Other definitions for years that I've seen before include "Units of time" , "Ages" , "12 month periods" , "Age" , "Long periods" .)