Crossword Genius

Viola's love for a lieutenant (4)

Ross

I believe the answer is:

alto

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'viola's' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'love for a lieutenant' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'for' says to put letters next to each other (I've seen this in other clues).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'a'+'lt'='alt'
'o' put after 'alt' is 'ALTO'.

Can you help me to learn more?

(Other definitions for alto that I've seen before include "One who sings" , "The highest adult male singing-voice" , "The highest adult male singing voice" , "High male singing voice" , "Male singer" .)

I've seen this clue in the Evening Standard.
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