Crossword Genius

Serious musical ability: something birds build up (7)

Ross

I believe the answer is:

earnest

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Sun.
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