Plate of lamb for one that’s upset (6)
I believe the answer is:
lamina
'plate' is the definition.
(anatomical term for a plate or layer)
'lamb for one that's upset' is the wordplay.
'lamb for one' becomes 'animal' (lamb is a kind of animal).
'that's upset' is a reversal indicator.
'animal' in reverse letter order is 'LAMINA'.
'of' is the link.
(Other definitions for lamina that I've seen before include "Flat plate" , "Thin plate or scale" , "Thin plate or layer" .)