Payment demanded by the letter (6)
I believe the answer is:
rental
'payment' is the definition.
(I've seen this before)
'demanded by the letter' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'letter' could be 'en' (I've seen this in other clues) and 'en' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rental that I've seen before include "To do with a letting" , "4 across (anag)" , "Payment to landlord" , "Payment for use -- antler (anag)" , "Hire fee" .)