Set on climbing (3)
I believe the answer is:
gel
'set' is the definition.
(both can mean to harden)
'on climbing' is the wordplay.
'on' becomes 'leg' (cricket term, fielding positions).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'leg' in reverse letter order is 'GEL'.
(Other definitions for gel that I've seen before include "Leg (anag.)" , "Capital preparation" , "shock application?" , "Preparation used in setting hair" , "Jelly-like substance for leg" .)