Sound man who is sincere (7)
I believe the answer is:
earnest
'is sincere' is the definition.
(synonyms)
'sound man' is the wordplay.
'sound' becomes 'ear' (I've seen this in other clues).
'man' becomes 'nest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ear'+'nest'='EARNEST'
'who' is the link.
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Sincere; enthusiastic" , "Serious - advance" , "Fervent - down payment" , "Intensely serious; token" , "Pledge - far from frivolous" .)