Let - pass (6)
I believe the answer is:
permit
I believe this clue is a double definition.
'let' is the first definition.
(I know that permit is a more specific form of let)
'pass' is the second definition.
(both can mean to allow)
(Other definitions for permit that I've seen before include "Allow, authorise" , "Licence -- license" , "Let; enabling document" , "Let - pass" , "Warrant" .)