Cat to criticise the king (7)
I believe the answer is:
panther
'cat' is the definition.
(I know that panther is a type of Panthera pardus)
'criticise the king' is the wordplay.
'criticise' becomes 'pan' (informal term meaning to criticise heavily).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'pan'+'the'+'r'='PANTHER'
'to' acts as a link.
(Other definitions for panther that I've seen before include "Black leopard" , "kind of feline" , "Big cat" , "animal!" , "Then rap the large black cat" .)