Gel (6)
'gel' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for harden that I've seen before include "Grow more firm" , "Anneal - solidify" , "Set; become tougher" , "Small fly" , "Become firm and solid" .)
'gel' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for congeal that I've seen before include "Curdle" , "Cool and solidify" , "Become solid" , "Become gelatinous" , "Set - go clean (anag)" .)