I, to Claudius (3)
'i to 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 "One's self-image" , "See 21 Down" , "Aspect of the personality" , "Inner self" , "(Self-assertive) personality" .)
(Other definitions for one that I've seen before include "If upper-class, perhaps I" , "1" , "Smallest whole number" , "Figure" , "Leading cardinal" .)