The most common and most likely answer to this crossword clue is the 7 letter word REGIONS
I've seen this crossword clue in The Daily Telegraph
R
E
G
I
O
N
S
More clues leading to the result REGIONS
Buddy explains!

The straight part
We know that "realms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case realms = regions.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does REGIONS mean?
REGIONS noun- Any considerable and connected part of a space or surface; specifically, a tract of land or sea of considerable but indefinite extent; a country; a district; in a broad sense, a place without special reference to location or extent but viewed as an entity for geographical, social or cultural reasons.
- An administrative subdivision of a city, a territory, a country.
- The inhabitants of a region or district of a country.
- A place in or a part of the body in any way indicated.
- Place; rank; station; dignity.
- The space from the earth's surface out to the orbit of the moon: properly called the elemental region.
- (definite: the regions) the rest of the country excluding the capital city or metropolitan region