Set of angels (3)
I believe the answer is:
gel
'set' is the definition.
(both can mean to harden)
'of angels' is the wordplay.
'of' indicates the answer is hidden within the clue.
'GEL' can be found hidden inside 'angels'.
(Other definitions for gel that I've seen before include "Hair treatment" , "Hair styling substance" , "Come together, take shape (colloq.)" , "Viscid substance" , "well-spoken deb?" .)