Shoot up to highest (3)
I believe the answer is:
pot
'shoot' is the definition.
(thesaurus)
'up to highest' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to highest' becomes 'top' (I've seen this before).
'top' reversed gives 'POT'.
(Other definitions for pot that I've seen before include "Cannabis - container - beer belly" , "Street name for marijuana" , "Container for 23" , "Kitty; marijuana" , "Cooking vessel or slang marijuana" .)