Portable shelter sheltering an occupant (6)
I believe the answer is:
'occupant' is the definition.
(tenant is a kind of occupant)
'portable shelter sheltering an' is the wordplay.
'portable shelter' becomes 'tent' (I've seen this before).
'sheltering' indicates putting letters inside.
'tent' going around 'an' is 'TENANT'.
(Other definitions for tenant that I've seen before include "Person renting property" , "He pays rent for lodgings" , "Anne Bronte's novel 'The ...... of Wildfell Hall'" , "Lessee" , "one paying rent" .)