The most common and most likely answer to this crossword clue is the 11 letter word INTERCOURSE
I've seen this crossword clue in Financial Times
I
N
T
E
R
C
O
U
R
S
E
Buddy explains!
The straight part
We know that "Sex" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sex = intercourse.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 INTERCOURSE mean?
INTERCOURSE noun- Communication, conversation.
- Dealings between countries.
- Dealings with people, including commerce and trade.
- Sexual intercourse usually involving humans.
- To have sexual intercourse.