Fool -- gangster to achieve victory over king (8)
I believe the answer is:
hoodwink
'fool' is the definition.
(thesaurus)
'gangster to achieve victory over king' is the wordplay.
'gangster' becomes 'hood' (I've seen this before).
'to achieve victory' becomes 'win' (I've seen this before).
'over' is a charade indicator (letters next to each other).
'king' becomes 'k'.
'hood'+'win'+'k'='HOODWINK'
(Other definitions for hoodwink that I've seen before include "Put wool over eyes of" , "Mislead" , "Con" , "Deceive, fool" , "Deceive, trick" .)