The most common and most likely answer to this crossword clue is the 4 plus 8 letter word WIFE SWAPPING
W
I
F
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S
W
A
P
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G
Buddy explains!
The straight part
We know that "Swinger’s activity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swinger’s activity = wife swapping.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.