Way to hold speed back — brake (6)
I believe the answer is:
retard
'brake' is the definition.
(thesaurus)
'way to hold speed back' is the wordplay.
'way' becomes 'rd' (short for road).
'to hold' indicates putting letters inside.
'speed' becomes 'rate' (the speed at which something happens).
'back' says the letters should be written in reverse.
'rate' in reverse letter order is 'etar'.
'rd' going around 'etar' is 'RETARD'.
(Other definitions for retard that I've seen before include "Hold back, slow progress" , "Keep back, make slow" , "Check" , "Impede like trader" , "Darted about to hold back or delay" .)