Sincere-sounding fellow (7)
I believe the answer is:
earnest
'sincere' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Sincere; determined" , "Eager, sincere" , "Seriously eager" , "Solemn; pledge" , "Intensely serious; token" .)