Voice in rational tones (4)
I believe the answer is:
alto
'voice' is the definition.
(alto is a kind of voice)
'in rational tones' is the wordplay.
'in' says the answer is hidden in the clue.
'ALTO' can be found hidden inside 'rational tones'.
(Other definitions for alto that I've seen before include "Kind of singing voice, male or female" , "Male singer" , "Unisex vocal range" , "Low female singing voice" , "Counter-tenor or contralto" .)