Top up a vessel (3)
I believe the answer is:
pot
'a vessel' is the definition.
(type of container)
'top up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'top' backwards is 'POT'.
(Other definitions for pot that I've seen before include "Cooking vessel, plant container and street drug" , "Kitchen utensil or slang cannabis" , "Ceramic container" , "Ceramic vessel" , "...Karenina(Tolstoy)" .)