Check reversing speed in road (6)
I believe the answer is:
retard
'check' is the definition.
(thesaurus)
'reversing speed in road' is the wordplay.
'reversing' shows that the letters should be reversed in order.
'speed' becomes 'rate' (rate is a kind of speed**).
'in' indicates putting letters inside.
'road' becomes 'rd' (abbreviation).
'rate' backwards is 'etar'.
'etar' going inside 'rd' is 'RETARD'.
(Other definitions for retard that I've seen before include "Hold back, slow progress" , "Hold back -- trader (anag)" , "Hold back (development)" , "Delay effect of" , "Detain" .)