Main character is power mad (6)
I believe the answer is:
pirate
'main character' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'power mad' is the wordplay.
'power' becomes 'p' (symbol in physics).
'mad' becomes 'irate' (I've seen this in another clue).
'p'+'irate'='PIRATE'
'is' is the link.
Can you help me to learn more?
(Other definitions for pirate that I've seen before include "Seaborne robber" , "Teach, perhaps" , "operating illegally" , "Publish without permission" , "Robber on the high seas" .)