Allow Harold to be mortal (6)
I believe the answer is:
lethal
'be mortal' is the definition.
(I've seen this before)
'allow harold' is the wordplay.
'allow' becomes 'let' (synonyms).
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'let'+'hal'='LETHAL'
'to' acts as a link.
(Other definitions for lethal that I've seen before include "Murderous" , "Deadly, potentially fatal" , "Terminal" , "Death-dealing" , "very nasty indeed" .)