Difficult task to drink up measure inside (6)
I believe the answer is:
killer
'difficult task' is the definition.
(killer can mean a difficult task or situation)
'drink up measure inside' is the wordplay.
'drink' becomes 'kir' (type of cocktail).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'measure' becomes 'ell' (historical unit of length).
'inside' means one lot of letters goes inside another.
'ell' in reverse letter order is 'lle'.
'kir' going around 'lle' is 'KILLER'.
'to' acts as a link.
(Other definitions for killer that I've seen before include "One murdering" , "Extremely impressive" , "Slayer" , "Murderer and bees" , "stupendous!" .)