Operatic role thought to release energy (3)
I believe the answer is:
ida
'operatic role' is the definition.
(Princess Ida in the Gilbert and Sullivan opera)
'thought to release energy' is the wordplay.
'thought' becomes 'idea' (synonyms).
'to release' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'idea' with 'e' taken away is 'IDA'.
(Other definitions for ida that I've seen before include "Industry development group" , "Cretan mount" , "The girl for Dai" , "lady" , "- Rubinstein, Russian dancer, actress" .)