Cheer up Harold when half-cut (3)
I believe the answer is:
rah
'cheer' is the definition.
(rah is a triumphant cry)
'up harold when half-cut' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'when half-cut' means to take half the letters.
'harold' with half the letters taken is 'har'.
'har' reversed gives 'RAH'.
(Other definitions for rah that I've seen before include "Cheer of approval" , "whoopee!" .)