Key Largo's ultimate voice (4)
I believe the answer is:
alto
'voice' is the definition.
(alto is a kind of voice)
'key largo's ultimate' is the wordplay.
'key' becomes 'alt' (I've seen this in other clues).
'ultimate' says to take the final letters.
The final letter of 'largo' is 'o'.
'alt'+'o'='ALTO'
(Other definitions for alto that I've seen before include "Key old > male who scores high" , "Lowest female singing voice" , "Singing range" , "Male singer" , "Voice part" .)