The one cast irrevocably? (3)
I believe the answer is:
die
'the one cast' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for die that I've seen before include "Device that stamps design on metal" , "one's cast" , "Cease to exist" , "Craps cube" , "One of a pair of dice" .)