The most common and most likely answer to this crossword clue is the 7 letter word EQUERRY
I've seen this crossword clue in The Daily Telegraph
E
Q
U
E
R
R
Y
More clues leading to the result EQUERRY
- Key question about king’s personal attendant
- Royal attendant really blowing top, having to guard queen repeatedly
- Royal officer completely losing head over queens
- Finally state doubt about queen’s attendant
- He resolves query when monarch’s in residence
- English raise a doubt about right for royal attendant
- Drug problem involving king’s personal servant?
- Person serving Queen may see European question about resistance
- Attendant always accommodating queen on railway
- First to examine question of concern about King or Queen?
Buddy explains!

The straight part
We know that "Person serving Queen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case person serving queen = equerry.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 EQUERRY mean?
EQUERRY noun- An officer responsible for the care and supervision of the horses of a person of rank.
- A personal attendant to a head of state, a member of a royal family, or a national representative.