Unlawful occupant, stockier (8)
I believe the answer is:
squatter
This clue is a double definition.
'unlawful occupant' is the first definition.
'stockier' is the second definition.
(Other definitions for squatter that I've seen before include "Person occupying property without legal right" , "Someone who settles on land without right or title" , "Someone occupying property with no legal right" , "illegally occupying property" , "One occupying disused building or land" .)