Knight left once over the fate of a cat (6)
I believe the answer is:
ocelot
'a cat' is the definition.
(ocelot is a kind of cat)
'knight left once over the fate' is the wordplay.
'knight' becomes 'n' (chess).
'left' suggests deleting specific letters.
'over' says to put letters next to each other.
'the fate' becomes 'lot' ('lot' can be a synonym of 'fate').
'once' with 'n' taken away is 'oce'.
'oce'+'lot'='OCELOT'
'of' acts as a link.
(Other definitions for ocelot that I've seen before include "Nocturnal wild cat of Central and South America" , "Feline mammal" , "Feline of South and Central America" , "Type of American wildcat" , "Wildcat -- loo etc (anag)" .)