The most common and most likely answer to this crossword clue is the 12 letter word EXTRAMARITAL
I've seen this crossword clue in The Guardian
E
X
T
R
A
M
A
R
I
T
A
L
More clues leading to the result EXTRAMARITAL
Buddy explains!
The straight part
We know that "Adulterous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case adulterous = extramarital.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 EXTRAMARITAL mean?
EXTRAMARITAL adjective- Taking place outside marriage.
- Adulterous.