Like a lion (7)
I believe the answer is:
'like a lion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for leonine that I've seen before include "Like the king of the beasts" , "Relating to a carnivore or to certain popes" , "As a maned animal" , "Catty" , "Like a big cat" .)