Payment letter demands material about conservationists (6)
I believe the answer is:
rental
'payment' is the definition.
(I've seen this before)
'material about conservationists' is the wordplay.
'material' becomes 'real' (associated in meaning).
'about' is an insertion indicator.
'conservationists' becomes 'nt' (abbreviation for National Trust).
'real' enclosing 'nt' is 'RENTAL'.
'letter demands' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rental that I've seen before include "Hire fee" , "Cost of hire" , "Learnt (anag) -- lease" , "What's offered by a letter" , "Payment to landlord" .)