One metre down, plant foolish hoax (8,3)
I believe the answer is:
Here is my best explanation:
'hoax' is the definition.
(Piltdown man is an example)
'one metre down plant foolish' is the subsidiary indication.
'one' becomes 'i' (roman numeral).
'metre' becomes 'm' ('m' can be a synonym of 'metre').
'foolish' indicates an anagram.
'imdownplant' is an anagram of 'piltdownman'.
(Other definitions for piltdown man that I've seen before include "Hoax purporting to be fossil hominid", "Pithecanthropus Dawsonii", "1912 Sussex fossil hoax - not limp wand (anag)" and "Fake fossil".)