The most common and most likely answer to this crossword clue is the 12 plus 3 letter word EXTRAMARITAL SEX
E
X
T
R
A
M
A
R
I
T
A
L
S
E
X
Buddy explains!
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.