Monthly expense
'monthly expense' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rent that I've seen before include "One due a letter" , "Landlord's due - that's torn it" , "Let out - torn" , "Tear; lease" , "Payment for lodging etc" .)
(Other definitions for gas that I've seen before include "See 2" , "eg Helium" , "THIRD PART OF 23" , "Something that is neither solid not liquid" , "Wind" .)