Supplier of drink left in glasses? (6)
I believe the answer is:
oporto
'supplier of drink' is the definition.
(Oporto in Portugal is an exporter of port wine)
'left in glasses?' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'in' is an insertion indicator.
'glasses?' becomes 'oo' (OO looks like spectacles).
'port' inserted inside 'oo' is 'OPORTO'.
(Other definitions for oporto that I've seen before include "European city" , "Iberian location" , "Portugal's second city" , "City near the mouth of the River Douro" , "Portuguese wine trade centre" .)