Grumpy companion, underhand, about to go up (6)
I believe the answer is:
sleepy
'grumpy companion' is the definition.
(Seven Dwarfs)
'underhand about to go up' is the wordplay.
'underhand' becomes 'sly' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'to go' becomes 'pee' (both can mean to urinate).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'pee' written backwards gives 'eep'.
'sly' placed around 'eep' is 'SLEEPY'.
(Other definitions for sleepy that I've seen before include "Lethargic" , "Ready to nod off" , "over-tired" , "Doc's nodding pal" , "One of the Seven Dwarfs" .)