They have made a spelling mistake. The advert should read "One room definitely available for rent, but also, possibly the whole house".
Just to ensure you know the meaning of both words:
Defiant means to stubbornly refuse to do something. For example, a child may defiantly refuse to brush their teeth.
Definite means that something is certain, I.E. it will happen. So "definitely" used within the advert means that there is certainly a room for rent in the house.