Become solid
I think it could be either:
(Other definitions for set that I've seen before include "Customary < scenery < a TV < series" , "Scenery for play or film" , "decline" , "Hardened" , "Major unit of tennis match" .)
'become solid' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gel that I've seen before include "Hair styling substance" , "Viscid substance" , "Upstage" , "Hair-dressing preparation" , "Hair cosmetic" .)