Part of angel is set (3)
I believe the answer is:
gel
'set' is the definition.
(both can mean to harden)
'part of angel is' is the wordplay.
'part of' indicates a hidden word.
'GEL' is hidden in the letters of 'angel is'.
(Other definitions for gel that I've seen before include "Take form" , "Preparation used in setting hair" , "shock treatment" , "Leg it back to get some for your hair" , "Hair-dressing preparation" .)