Crossword Genius

In audition, change bird -- 15 will be at it one day (5,4)

Ross

I believe the answer is:

altar rail

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

'15 will be at it one day' is the definition.
I can't judge whether this definition defines the answer.

'in audition change bird' is the wordplay.
'in audition' indicates a 'sounds like' (homophone) clue (audition can mean hearing).
'change' becomes 'alter' ('alter' can be a synonym of 'change').
'bird' becomes 'rail' (rail is a kind of bird).
'alter' is a homophone of 'altar'.
'altar'+'rail'='ALTAR RAIL'

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I've seen this clue in The Telegraph.
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