The most common and most likely answer to this crossword clue is the 7 letter word PERVERT
I've seen this crossword clue in The Guardian
P
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More clues leading to the result PERVERT
Buddy explains!

The straight part
We know that "sexual deviant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sexual deviant = pervert.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 PERVERT mean?
PERVERT noun- One who has been perverted; one who has turned to error; one who has turned to a twisted sense of values or morals.
- A person whose sexual habits are not considered acceptable.
- To turn another way; to divert.
- To corrupt; to cause to be untrue; corrupted or otherwise impure
- To misapply, misuse, use for a nefarious purpose
- To misinterpret designedly.
- To become perverted; to take the wrong course.