How mortgage may be secured free of charge (2,3,5)
I believe the answer is:
on the house
'free of charge' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
(Other definitions for on the house that I've seen before include "At the publican's expense" , "Where satellite dish may be" , "Free" , "With the management's compliments" , "Where the roofer may be working" .)