Wicked fish -- it swallows a knight (9)
I believe the answer is:
brilliant
'wicked' is the definition.
(I know that wicked can be written as brilliant)
'fish it swallows a knight' is the wordplay.
'fish' becomes 'brill' (brill is a kind of fish).
'swallows' indicates putting letters inside.
'knight' becomes 'N' (chess abbreviation).
'a'+'n'='an'
'it' placed around 'an' is 'iant'.
'brill'+'iant'='BRILLIANT'
(Other definitions for brilliant that I've seen before include "Exceptionally clever" , "Glittering" , "Magnificent" , "Very bright or exceptionally talented" , "Fantastic!" .)