Payment letter demands material about conservationists (6)
I believe the answer is:
Here is my best explanation:
'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' going around 'nt' is 'RENTAL'.
'letter demands' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.