Set a broken leg (3)
I believe the answer is:
gel
'set' is the definition.
(both can mean to harden)
'a broken leg' is the wordplay.
'a broken' is an anagram indicator.
'leg' is an anagram of 'GEL'.
(Other definitions for gel that I've seen before include "Begin to work" , "Leg (anag.)" , "Sticky substance" , "posh lass" , "Become jelly-like" .)