Undesirable people (4-4)
(Other definitions for riff-raff that I've seen before include "rubbish" , "Disreputable people" , "Rabble" , "Hoi polloi" , "Dregs of society" .)
'undesirable people' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for riffraff that I've seen before include "Crowd regarded with disdain" , "Dregs of society" , "Rubbish" , "Rabble, crowd of unruly people" , "Disreputable people, rabble" .)