Conservation organisation (8,5)
I believe the answer is:
national trust
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">conservation organisation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for national trust that I've seen before include "Preserving organisation" , "Heritage conservation charity" , "Society concerned with preservation" , "Conservation organisation" , "One of Britain's largest land-owners" .)