Eighteen holes, then buy this at the nineteenth? (5)
I believe the answer is:
round
This clue is a double definition.
'eighteen holes' is the first definition.
(a round of golf includes eighteen holes)
'buy this at the nineteenth?' is the second definition.
(round of drinks bought at 'the 19th hole' ie a pub)
'then' is the link.
(Other definitions for round that I've seen before include "Regular route for a sentry" , "Piece of ammunition" , "A partsong in which voices follow each other" , "regular series" , "Lap" .)