Mineral from old Roman watering-hole (8)
I believe the answer is:
cinnabar
'mineral' is the definition.
(I've seen this before)
'old roman watering-hole' is the wordplay.
'old roman' becomes 'cinna' (Roman politician).
'watering hole' becomes 'bar' (I've seen this before).
'cinna'+'bar'='CINNABAR'
'from' is the link.
(Other definitions for cinnabar that I've seen before include "Heavy red mineral containing mercury" , "Bright red mineral containing mercury" , "Red mercury sulphide" , "Brian can make vermilion pigment" .)