Unlawful occupant (8)
I believe the answer is:
squatter
'unlawful occupant' is the definition.
This is the entire clue.
(Other definitions for squatter that I've seen before include "Illegal occupier of empty property" , "Less tall and slim" , "Illegal occupant of dwelling" , "Someone who settles on land without right or title" , "Quartets (anag) -- illegal occupant" .)