One metre down, plant foolish hoax (8,3)
I believe the answer is:
piltdown man
'hoax' is the definition.
(Piltdown man is an example)
'one metre down plant foolish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'metre' becomes 'm' ('m' can be a synonym of 'metre').
'foolish' is an anagram indicator.
'i'+'m'+'down'+'plant'='imdownplant'
'imdownplant' anagrammed gives 'PILTDOWN MAN'.
(Other definitions for piltdown man that I've seen before include "Pithecanthropus Dawsonii" , "old fraud!" , "1912 Sussex fossil hoax - not limp wand (anag)" , "Hoax purporting to be fossil hominid" , "Fake fossil" .)