Part for Maria Callas to study endlessly (5)
I believe the answer is:
tosca
'part for maria callas' is the definition.
(opera part performed by Maria Callas)
'to study endlessly' is the wordplay.
'study' becomes 'scan' (I've seen this before).
'endlessly' means to remove the last letter.
'scan' with its last letter taken away is 'sca'.
'to'+'sca'='TOSCA'
(Other definitions for tosca that I've seen before include "Opera heroine who leaps to her death" , "Coats off for Puccini's opera" , "Leaping opera heroine" , "role for diva" , "Puccini's Roman heroine" .)