Impediment (3)
'impediment' is the definition.
('let' could historically mean a hindrance)
This is all the clue.
(Other definitions for let that I've seen before include "Permitted" , "Permit; lease" , "Allow the obstruction" , "Award (a contract)" , "Disappoint or fail to support - ... down" .)
'impediment' is the definition.
(I know that impediment can be written as hindrance)
This is the entire clue.
(Other definitions for hindrance that I've seen before include "Delay to overcome" , "barrier" , "Nicer hand (anag.)" , "Something inhibiting action" , "Impediment" .)