Permit Prince Henry to be deadly (6)
I believe the answer is:
lethal
'to be deadly' is the definition.
(synonyms)
'permit prince' is the wordplay.
'permit' becomes 'let' (synonyms).
'prince' becomes 'hal' (Shakespearean nickname of Henry V when a prince).
'let'+'hal'='LETHAL'
'henry' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lethal that I've seen before include "Deadly or mortal" , "Liable to cause death" , "Capable of causing death" , "Death-dealing" , "very nasty indeed" .)