"Yes, indeed"
(Other definitions for itis that I've seen before include "Suffix signifying inflammation" , "Suffix denoting inflammation" .)
I believe this clue is a double definition.
'yes' is the first definition.
(I've seen this before)
'indeed' is the second definition.
(I've seen this before)
(Other definitions for absolutely that I've seen before include "I wholly agree!" , "You bet!" , "Utterly" , "Dead" , "Totally" .)