The most common and most likely answer to this crossword clue is the 13 letter word DOUBLE-BOOKED
I've seen this crossword clue in Financial Times
D
O
U
B
L
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B
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D
Buddy explains!
The straight part
We know that "Having a foursome in the honeymoon suite" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case having a foursome in the honeymoon suite = double-booked.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 DOUBLE-BOOKED mean?
DOUBLE-BOOKED verb- To book or reserve something for two people simultaneously.
- To make arrangements to do two things at the same time, such that it is impossible to keep both arrangements.