Allow Henry to be a killer! (6)

I believe the answer is:

lethal

I'm a little stuck... Click here to teach me more about this clue!

'to be a killer' is the definition.
I can't tell whether this definition defines the answer.

'allow henry' is the wordplay.
'allow' becomes 'let' (synonyms).
'henry' becomes 'hal' (short for Henry).
'let'+'hal'='LETHAL'

Can you help me to learn more?

(Other definitions for lethal that I've seen before include "Vicious" , "--- Weapon, Mel Gibson film" , "Murderous" , "Deadly or mortal" , "Noxious" .)

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