Slow, steep A-road (6)
I believe the answer is:
retard
'slow' is the definition.
(synonyms)
'steep a-road' is the wordplay.
'steep' becomes 'ret' (I've seen this in another clue).
'road' becomes 'rd' (abbreviation for road).
'ret'+'a'+'rd'='RETARD'
(Other definitions for retard that I've seen before include "Slow down growth" , "Slow the progress of, delay" , "Slow down for trader" , "Hinder progress" , "Hold back, slow progress" .)