It's the height of Violet and Ena combined (9)
I believe the answer is:
elevation
'it's the height' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for elevation that I've seen before include "Height above a given level" , "Drawing of the external face of a building" , "Leave it on for altitude of a place" , "Height - promotion" , "scale drawing" .)