Costly to many a listener (4)
I believe the answer is:
dear
'costly' is the definition.
(synonyms)
'many a listener' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'a listener' becomes 'ear' (body part which hears or listens).
'd'+'ear'='DEAR'
'to' is the link.
(Other definitions for dear that I've seen before include "Darling" , "Expensive or loved" , "Expensive - address" , "Highly priced" , "At a high cost" .)