There are multiple definitions - click on the translation that matches your context.

fremleie

verb

full

underlet

verb

Definitions of underlet

(of a tenant or a lessee) to sublet property (=to rent property held on a lease to another person)

The Tenant agrees not to underlet the whole of the Premises or underlet a Permitted Part of the Premises without the consent of the Landlord.

to lease property for less than its true value

All of my properties are underlet.