Concrete ran smoothly with nothing to stop it (5)
I believe the answer is:
solid
'concrete' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for solid that I've seen before include "Idols (anag.)" , "Of substance" , "Hard and compact" , "Reliable; not hollow" , "Not hollow, the same throughout" .)