Top up container (3)
I believe the answer is:
pot
'container' is the definition.
(I know that pot is a type of container)
'top up' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'top' back-to-front is 'POT'.
(Other definitions for pot that I've seen before include "Slangy cannabis" , "Sink (a snooker ball)" , "Container for 23" , "Cooking vessel, plant container and street drug" , "Ceramic vessel" .)