Callas's able somehow to retain musical sense with purest tone (2,5,2,1,4)

'with purest tone' is the definition.
(as clear as a bell describes pure or perfect sound)

'callas's able somehow to retain musical sense' is the wordplay.
'somehow' is an anagram indicator.
'to retain' means one lot of letters goes inside another.
'musical sense' becomes 'ear' (I've seen this in another clue).
'callass'+'able'='callassable'
'callassable' with letters rearranged gives 'asclasabell'.
'asclasabell' enclosing 'ear' is 'AS CLEAR AS A BELL'.