Top up kitty (3)
I believe the answer is:
pot
'kitty' is the definition.
(I've seen this before)
'top up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'top' reversed gives 'POT'.
(Other definitions for pot that I've seen before include "Somewhere to go" , "Street name for cannabis" , "Saucepan" , "Jar, cannabis" , "Grass" .)