Crossword Genius

One sax used in national tour (4)

Ross

I believe the answer is:

alto

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

'one sax' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'in national tour' is the wordplay.
'in' indicates a hidden word.
'ALTO' is hidden within 'national tour'.

'used' acts as a link.

Can you help me to learn more?

(Other definitions for alto that I've seen before include "Second of four parts (in singing)" , "Voice part" , "The highest adult male singing voice" , "Singer's hidden inside chemical toilet" , "Singing range" .)

I've seen this clue in The Sun.
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