Dreadful torrent, flowing with no resistance (6)
I believe the answer is:
rotten
'dreadful' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rotten that I've seen before include "Nasty" , "Informally, very much" , "'Gone bad, decayed (6)'" , "unpleasant" , "Falling to pieces from age or morally contemptible" .)