Crystal meth inside stay out! (8)
I believe the answer is:
amethyst
'crystal' is the definition.
(amethyst is a kind of crystal)
'meth inside stay out' is the wordplay.
'inside' is an insertion indicator.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'stay' anagrammed gives 'ayst'.
'meth' inserted within 'ayst' is 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "Variety of quartz" , "Something precious" , "One showing sparkling facets" , "Purple quartz used in jewellery" , "Type of quartz" .)