Set to give a leg up (3)
I believe the answer is:
gel
'set' is the definition.
(I've seen this before)
'a leg up' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'leg' backwards is 'GEL'.
'to give' is the link.
(Other definitions for gel that I've seen before include "Solidify" , "Substance to fix hair in place" , "shock application?" , "Mousse" , "Semi-solid" .)