Unauthorised occupant (8)
I believe the answer is:
squatter
'unauthorised occupant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for squatter that I've seen before include "Comparatively dumpy person" , "Someone occupies land illegally" , "person in illegal occupation" , "Someone who settles on land without right or title" , "Illegal occupier of empty property" .)