Portable shelter sheltering an occupant (6)
I believe the answer is:
Here is my best explanation:
'occupant' is the definition.
(tenant is a kind of occupant)
'portable shelter sheltering an' is the subsidiary indication.
'portable shelter' becomes 'tent' (I've seen this before).
'sheltering' is an insertion indicator.
'tent' going around 'an' is 'tenant'.
(Other definitions for tenant that I've seen before include "Someone who pays rent", "He pays rent for lodgings", "Renting occupant", "Person renting property", "Occupant of rented property", "he or she rents", "Anne Bronte's novel 'The ...... of Wildfell Hall'", "one paying rent", "A rent payer" and "Rent-payer".)