A drink for a horse (6)
I believe the answer is:
chaser
I believe this is a double definition.
'a drink' is the first definition.
(as in a shot and chaser)
'horse' is the second definition.
(I've seen this before)
'for a' is the link.
(Other definitions for chaser that I've seen before include "Beer after whisky, say (or vice versa)" , "Yellow-Dog Dingo?" , "Drink that follows immediately after another one" , "second drink" , "Follow-up drink" .)