The most common and most likely answer to this crossword clue is the 15 letter word PARLIAMENTARIAN
I've seen this crossword clue in The Times
P
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More clues leading to the result PARLIAMENTARIAN
- Owl, might one say: once an opponent of the king?
- Average, struggling Inter Milan twice letting in a sitter?
- Roundhead, Martin, a real pain possibly
- The old man approaching air terminal wobbling, an elected representative?
- Person at home in house, having taken a seat
- One formally engaged in a diet?
- One sitting in normal section of joint not good with an air that’s suspect
- Politician playing part in arena, holding revolutionary post
Buddy explains!

The straight part
We know that "Owl, might one say/once an opponent of the king" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case owl, might one say/once an opponent of the king = parliamentarian.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 PARLIAMENTARIAN mean?
PARLIAMENTARIAN noun- A member of a parliament, congress or an elected national legislative body of another name.
- A person well-versed in parliamentary procedure.
- An officer in most legislative bodies charged with being well-versed in the parliamentary rules of that legislative house, and whose rulings are taken as authoritative, to be appealed only to the whole of the house itself under special rules.
- Of or relating to a parliament; favouring the establishment of a parliament.
- Of or relating to the Parliamentarians (supporters of the parliament during the English Civil War (1642–1651)).