Singer is ideal Tosca? Not entirely (4)
I believe the answer is:
alto
'singer' is the definition.
(alto is a kind of singer)
'ideal tosca? not entirely' is the wordplay.
'not entirely' indicates the answer is hidden within the clue.
'ALTO' is hidden within 'ideal tosca'.
'is' acts as a link.
(Other definitions for alto that I've seen before include "Lowest female singing voice" , "Singer's hidden inside chemical toilet" , "The highest adult male singing voice" , "One who sings" , "A singing voice or pitch" .)