Become firm
I believe the answer is:
gel
'become firm' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gel that I've seen before include "Solidify" , "Leg it and get it for your hair, say" , "Hair cosmetic" , "Jelly-like substance for leg" , "Congeal - hair dressing" .)