"I" for Claudius (3)
I believe the answer is:
ego
'i for claudius' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ego that I've seen before include "Sense of self, conceit" , "Conscious mind" , "One's self-image" , "See 21 Down" , "Inner self" .)