Great madness (11)
I believe the answer is:
megalomania
'great madness' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'great madness' is the wordplay.
I cannot really understand how this works, but
'great' could be 'mega' (I've seen this in other clues) and 'mega' is found in the answer.
'madness' could be 'mania' (I've seen this before) and 'mania' is located in the answer.
The remaining letters 'lo' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for megalomania that I've seen before include "Illness" , "Folie de grandeur" , "Passion for grandeur" , "Delusion about one's own power" .)