Tarred in order to slow down (6)
I believe the answer is:
retard
'slow down' is the definition.
(I know that slow down can be written as retard)
'tarred in order' is the wordplay.
'in order' indicates anagramming the letters (the letters in a new order).
'tarred' with letters rearranged gives 'RETARD'.
'to' acts as a link.
(Other definitions for retard that I've seen before include "Hold back (development)" , "Hinder progress" , "Detain" , "Impede like trader" , "Darted about to hold back or delay" .)