The most common and most likely answer to this crossword clue is the 11 letter word MOMENTARILY
I've seen this crossword clue in The Guardian
M
O
M
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N
T
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More clues leading to the result MOMENTARILY
Buddy explains!
The straight part
We know that "Briefly in Britain/Briefly in Britain/very soon in the US/very soon in the US" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case briefly in britain/briefly in britain/very soon in the us/very soon in the us = momentarily.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 MOMENTARILY mean?
MOMENTARILY adverb- (manner) In a momentary manner; for a moment or instant.
- (duration) In a moment or very soon; any minute now, any time now.
- Progressively; moment by moment.