Gem that could make them stay (8)
I believe the answer is:
amethyst
'gem' is the definition.
(amethyst is a kind of gem)
'could make them stay' is the wordplay.
'could make' is an anagram indicator.
'them'+'stay'='themstay'
'themstay' anagrammed gives 'AMETHYST'.
'that' is the link.
(Other definitions for amethyst that I've seen before include "Something precious" , "Transparent bluish quartz" , "Type of quartz" , "Purple gem" , "what's in ring" .)