Naturalist not working for heritage organisation (8,5)
I believe the answer is:
national trust
'heritage organisation' is the definition.
(I've seen this before)
'naturalist not working' is the wordplay.
'working' indicates anagramming the letters.
'naturalist'+'not'='naturalistnot'
'naturalistnot' is an anagram of 'NATIONAL TRUST'.
'for' is the link.
(Other definitions for national trust that I've seen before include "Preservation body" , "British institution for preserving buildings, monuments, gardens etc" , "One of Britain's largest land-owners" , "Heritage conservation charity" , "Conservationist organisation" .)