The most common and most likely answer to this crossword clue is the 7 plus 7 letter word ABRAHAM LINCOLN
I've seen this crossword clue in The Times
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More clues leading to the result ABRAHAM LINCOLN
- One great shot in the box by American player
- A scorer briefly crossing a sort of green, one at the White House once
- Union leader who met a theatrical end?
- Amendment of a bill can harm no president
- A great shot inside box by US player
- As crazy monarch, I’ll ban a president
- & 20d A coarse City supporter, leading advocate of freedom in speech
- As Republican president, I’ll ban a monarch after revolution
Buddy explains!
The straight part
We know that "one at the White House once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one at the white house once = abraham lincoln.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.