Retard; inhibit (6)
I believe the answer is:
hinder
I believe this clue is a double definition.
'retard' is the first definition.
(thesaurus)
'inhibit' is the second definition.
(I know that inhibit is a more specific form of the action hinder)
(Other definitions for hinder that I've seen before include "Delay progress, obstruct" , "Obstruct or hold back" , "Obstruct; nearer the back" , "Bow user" , "Obstruct, impede" .)