Allow Harold to be deadly (6)
I believe the answer is:
lethal
'be deadly' is the definition.
(synonyms)
'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 "Death-dealing" , "Able to cause death" , "describing a killer?" , "All the change can be deadly." , "Terminal" .)