It fixes stage up (3)
I believe the answer is:
gel
'it fixes' is the definition.
The definition suggests a singular noun which matches the answer.
'stage up' is the wordplay.
'stage' becomes 'leg' (a leg is a stage of a competition).
'up' is a reversal indicator (in down clue: letters go upwards).
'leg' backwards is 'GEL'.
Can you help me to learn more?
(Other definitions for gel that I've seen before include "stiffen" , "Hair-dressing preparation" , "Leg (anag.)" , "Become jelly-like" , "Mousse" .)