Central idea Iolanthe means to convey (5)
I believe the answer is:
theme
'central idea' is the definition.
(I've seen this in another clue)
'iolanthe means to convey' is the wordplay.
'THEME' is hidden within the letters.
However, I am uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for theme that I've seen before include "tune used in Enigma Variations?" , "He met (anag) - subject" , "sort of tune" , "subject,motif" , "Subject set for discussion" .)