A vessel to top up with drink (6)
I believe the answer is:
teapot
'a vessel' is the definition.
(teapot is a kind of vessel)
'top up with drink' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'with' says to put letters next to each other.
'drink' becomes 'tea' (tea is a kind of drink**).
'top' backwards is 'pot'.
'pot' after 'tea' is 'TEAPOT'.
'to' acts as a link.
(Other definitions for teapot that I've seen before include "Peat to brew leaves in" , "What mother perhaps holds" , "Hot drink dispenser" , "Vessel for brewing leaves" , "Spouted vessel" .)