Chums? Not right — cruel folk (6)
I believe the answer is:
fiends
'cruel folk' is the definition.
'chums? not right' is the wordplay.
'chums?' becomes 'friends' (chum is a kind of friend).
'not' suggests deleting specific letters.
'right' becomes 'r' (common abbreviation).
'friends' with 'r' taken out is 'FIENDS'.
(Other definitions for fiends that I've seen before include "Cruel people or evil spirits" , "Evil spirits or wicked people" , "Very wicked or cruel people" , "fanatics" , "Addicts; devotees" .)