Empire (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
'empire' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for realm that I've seen before include "Royal domain" , "Kingdom; field of interest" , "Kingdom, domain" , "King's domain" , "Area of interest/activity" .)