Thy meat's going off -- it's purple! (8)
I believe the answer is:
amethyst
'purple' is the definition.
(thesaurus)
'thy meat's going off' is the wordplay.
'going off' indicates anagramming the letters.
'thy'+'meats'='thymeats'
'thymeats' with letters rearranged gives 'AMETHYST'.
'it's' is the link.
(Other definitions for amethyst that I've seen before include "One showing sparkling facets" , "Semiprecious stone of violet or purple quartz" , "Type of quartz" , "Purple gem" , "Purple quartz, used as a gemstone" .)