For keys in the house, this must be paid (6)
I believe the answer is:
homage
'this must be paid' is the definition.
'homage' can be an answer for 'must' (thesaurus). I'm not sure about the remainder of the definition.
'for keys in the house' is the wordplay.
'for keys' becomes 'ag' (I can't explain this - if you can you should believe this answer much more).
'in the' indicates putting letters inside.
'house' becomes 'home' (homing is a kind of housing).
'ag' inserted within 'home' is 'HOMAGE'.
Can you help me to learn more?
(Other definitions for homage that I've seen before include "Due respect" , "Tribute, reverence" , "Things said as mark of respect" , "Formal expression of loyalty to a ruler - pay it" , "Publicly-shown respect" .)