Thin layer, round, neither vegetable nor mineral (6)
I believe the answer is:
animal
'neither vegetable nor mineral' is the definition.
(I've seen this before)
'thin layer round' is the wordplay.
'thin layer' becomes 'lamina' (I've seen this before).
'round' shows that the letters should be reversed in order.
'lamina' written backwards gives 'ANIMAL'.
(Other definitions for animal that I've seen before include "See 26" , "Man or beast" , "Living organism characterised by voluntary movement" , "Organism having life" , "Kind of instinct" .)