Listener starts to need expert solvers to be serious (7)
I believe the answer is:
'serious' is the definition.
(similar in meaning)
'listener starts to need expert solvers to' is the wordplay.
'listener' becomes 'ear' (I've seen this before).
'starts to' indicates taking the first letters.
The first letters of 'need expert solvers to' is 'nest'.
'be' is the link.
(Other definitions for earnest that I've seen before include "Sincere in intention" , "Sincere or over-serious" , "Serious, solemn" , "'Heartfelt, sincere (7)'" , "Serious or heartfelt" .)