Keen listener sent for replacement (7)
I believe the answer is:
earnest
'keen' is the definition.
(thesaurus)
'listener sent for replacement' is the wordplay.
'listener' becomes 'ear' (I've seen this before).
'for replacement' indicates anagramming the letters (I've seen this in other clues).
'sent' anagrammed gives 'nest'.
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "money involved in contract" , "Sincere; determined" , "Oscar Wilde wrote ''The importance of Being . . . . . . . '' (7)" , "art collector" , "Serious; pledge" .)