One singing in reverential tones (4)
I believe the answer is:
alto
'one singing' is the definition.
(thesaurus)
'in reverential tones' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALTO' is hidden within 'reverential tones'.
(Other definitions for alto that I've seen before include "Lowest female singing voice" , "Low female singing voice" , "Voice part" , "Woman's singing voice" , "One who sings" .)