I was using wrong the clamp() method.
For anyone with this same question, my code is something like this:
export var MaxHP = 5
onready var TheHP = MaxHP
onready var HP = clamp(TheHP, 0, MaxHP)
TheHP -= DP
HP = clamp(TheHP, 0, MaxHP)
TheHP = HP
if HP <= 0:
I don't know if is the optimums way, but it's works!