The most common and most likely answer to this crossword clue is the 6 letter word OBSESS
I've seen this crossword clue in Financial Times
O
B
S
E
S
S
More clues leading to the result OBSESS
- Love Elizabeth taking in second old hound
- Oscar and Elizabeth taking in small hound
- Keep thinking only about entering classes boxer set up
- Initially one shilling to enter Elizabeth I’s haunt?
- Honours son after accepting son’s rule
- Preoccupy managers by switching leaders
- To dominate, bosses about
- Be preoccupied with love and Elizabeth I having sex
- Be preoccupied about manager losing head over complaint
- Bungling bosses continuously fill the mind
Buddy explains!

The straight part
We know that "Preoccup" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preoccup = obsess.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 OBSESS mean?
OBSESS verb- (passive, constructed with "with") To be preoccupied with a single topic or emotion.
- To dominate the thoughts of someone.
- (construed with over) To think or talk obsessively about.