Occupier is not available in shelter (6)
I believe the answer is:
tenant
'occupier' is the definition.
(I know that tenant is a type of occupier)
'not available in shelter' is the wordplay.
'not available' becomes 'na' (abbreviation for not available).
'in' is an insertion indicator.
'shelter' becomes 'tent' (canvas shelter).
'na' put into 'tent' is 'TENANT'.
'is' acts as a link.
(Other definitions for tenant that I've seen before include "Rent-payer" , "his flat?" , "Leaseholder" , "Occupant of rented property" , "He pays rent for lodgings" .)