Allow Henry in terminal (6)
I believe the answer is:
lethal
'terminal' is the definition.
(thesaurus)
'allow henry' is the wordplay.
'allow' becomes 'let' ('let' can be a synonym of 'allow').
'henry' becomes 'hal'.
'let'+'hal'='LETHAL'
'in' is the link.
(Other definitions for lethal that I've seen before include "Terminal" , "Killing" , "Noxious" , "Liable to cause death" , "bringing about the end?" .)