Rage in its entirety (3)
I believe the answer is:
ire
'rage' is the definition.
(ire is intense rage)
'in its entirety' is the wordplay.
'in' says the answer is hidden in the clue.
'IRE' can be found hidden inside 'its entirety'.
(Other definitions for ire that I've seen before include "Anger, rage" , "Outrage" , "Annoyance, vexation" , "Intense anger" , "'Anger, wrath (3)'" .)