The most common and most likely answer to this crossword clue is the 6 letter word INFEST
I've seen this crossword clue in The Daily Telegraph
I
N
F
E
S
T
Buddy explains!

The straight part
We know that "Unwelcomely occupy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unwelcomely occupy = infest.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 INFEST mean?
INFEST noun- Hostility.
- To inhabit a place in unpleasantly large numbers; to plague, harass.
- (of a parasite) To invade a host plant or animal.
- Mischievous; hurtful; harassing.