Old Roman watering-hole provides a mineral (8)
I believe the answer is:
cinnabar
'a 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'
'provides' acts as a link.
(Other definitions for cinnabar that I've seen before include "Brian can make vermilion pigment" , "Heavy red mineral containing mercury" , "Red mercury sulphide" , "Bright red mineral containing mercury" .)