The most common and most likely answer to this crossword clue is the 6 letter word VOYEUR
I've seen this crossword clue in Independent
V
O
Y
E
U
R
Buddy explains!

The straight part
We know that "One who watches" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who watches = voyeur.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 VOYEUR mean?
VOYEUR noun- A person who derives sexual pleasure from secretly observing other people, especially when such people are engaged in some sexual activity.
- An obsessive observer of sensational or sordid subjects.