A gem of wine (4)
I believe the answer is:
ruby
This clue is a double definition.
'a gem' is the first definition.
(ruby is a kind of gem)
'wine' is the second definition.
(thesaurus)
'of' acts as a link.
(Other definitions for ruby that I've seen before include "Gemstone - shade of red" , "Girl's name - colour" , "Jack perhaps" , "Precious red gem" , "Deep red variety of corundum" .)