Set stage up (3)
I believe the answer is:
gel
'set' is the definition.
(both can mean to harden)
'stage up' is the wordplay.
'stage' becomes 'leg' (a leg is a stage of a competition).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'leg' backwards is 'GEL'.
(Other definitions for gel that I've seen before include "Preparation used in setting hair" , "Substance to fix hair in place" , "shock application?" , "Become jelly-like" , "Hair styling substance" .)