Serious listener sent astray (7)
I believe the answer is:
earnest
'serious' is the definition.
('earnest' can be similar in meaning to 'serious')
'listener sent astray' is the wordplay.
'listener' becomes 'ear' (I've seen this before).
'astray' indicates anagramming the letters.
'sent' anagrammed gives 'nest'.
'ear'+'nest'='EARNEST'
(Other definitions for earnest that I've seen before include "Serious - money deposit" , "Grave" , "Oscar Wilde wrote ''The importance of Being . . . . . . . '' (7)" , "Showing sincere and intense conviction" , "Solemn; pledge" .)