The most common and most likely answer to this crossword clue is the 5 letter word ROYAL
I've seen this crossword clue in The Times
R
O
Y
A
L
More clues leading to the result ROYAL
- Old Republican rejected song about king, for example
- Member of ruling family put up by Republican circle
- Splendid non-professional people without commission turned up
- Though no son, optimistic over a line of monarchs
- Stag would be faithful under a different leader
- Member of this family finds Republican not initially faithful
- Sail this before we commission standard navy marines
- Song originally on release when backing Prince?
- Ruler, having pledged allegiance, changed sides at the start
- Like a Prince of Troy, allegedly
Buddy explains!

The straight part
We know that "Member of ruling family" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case member of ruling family = royal.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 ROYAL mean?
ROYAL noun- A royal person; a member of a royal family.
- A standard size of printing paper, measuring 25 by 20 inches.
- A standard size of writing paper, measuring 24 by 19 inches.
- The Australian decimal currency intended to replace the pound in 1966; was changed to "dollar" before it was actually circulated.
- Any of various lycaenid butterflies.
- The fourth tine of an antler's beam.
- A stag with twelve points (six on each antler).
- In large sailing ships, square sail over the topgallant sail.
- An old English gold coin, the rial.
- A small mortar.
- In auction bridge, a royal spade.
- A tuft of beard on the lower lip.
- (campanology) Bell changes rung on ten bells.
- Of or relating to a monarch or their family.
- Having the air or demeanour of a monarch; illustrious; magnanimous; of more than common size or excellence.
- In large sailing ships, of a mast right above the topgallant mast and its sails.
- Free-for-all, especially involving multiple combatants.
- Used as an intensifier.