The most common and most likely answer to this crossword clue is the 11 letter word FORNICATION
I've seen this crossword clue in The Guardian
F
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Buddy explains!
The straight part
We know that "How’s your father" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how’s your father = fornication.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 FORNICATION mean?
FORNICATION noun- Sexual intercourse by people who are not married, or which is considered illicit in another way.
- Sexual intercourse in general; sex.