The most common and most likely answer to this crossword clue is the 6 plus 3 letter word SKINNY DIP
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More clues leading to the result SKINNY DIP
Buddy explains!
The straight part
We know that "swim in birthday suit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swim in birthday suit = skinny dip.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 SKINNY DIP mean?
SKINNY DIP verb- To swim naked.