I know it's incorrect king is cruel (4)
I believe the answer is:
sick
'cruel' is the definition.
(sick can mean cruel or sadistic)
'i know it's incorrect king' is the wordplay.
'i know it's incorrect' becomes 'sic' ('sic' is used to acknowledge an error in a quote).
'king' becomes 'K' (chess abbreviation).
'sic'+'k'='SICK'
'is' is the link.
(Other definitions for sick that I've seen before include "Bad" , "Ill; perverted" , "vomit" , "well? Hardly" , "Ill; nauseous" .)