I test one rotten piece of meat (8)
I believe the answer is:
noisette
'piece of meat' is the definition.
(I've seen this before)
'i test one rotten' is the wordplay.
'rotten' is an anagram indicator.
'i'+'test'+'one'='itestone'
'itestone' with letters rearranged gives 'NOISETTE'.
(Other definitions for noisette that I've seen before include "Some rolled meat" , "Small piece of boneless lamb" , "Hazelnut chocolate" , "Chocolate; piece of meat" , "Piece of rolled lamb (flavoured with hazel nuts?)" .)