Serious musical ability: something birds build up (7)
I believe the answer is:
earnest
'serious' is the definition.
(similar in meaning)
'musical ability something birds build up' is the wordplay.
I cannot quite understand how this works, but
'ability' could be 'ear' (ear is a kind of ability) and 'ear' is present in the answer.
The remaining letters 'nest' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for earnest that I've seen before include "Serious; down-payment" , "Sincere in intention" , "money put down" , "Serious - advance" , "Sneer at (anag) -- serious" .)