The most common and most likely answer to this crossword clue is the 4 letter word OXON
I've seen this crossword clue in Independent
O
X
O
N
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.
What does OXON mean?
OXON noun- Any organic compound derived from another in which a phosphorus-sulfur bond in the parent has been replaced by a phosphorus-oxygen bond in the derivative.