Feline in rage biting unfortunate child, nipper initially rolling over (4,3)
I believe the answer is:
lion cub
'feline' is the definition.
(I know that lion cub is a type of Panthera leo)
'rage biting unfortunate child nipper initially rolling over' is the wordplay.
'rage' becomes 'boil' (I've seen this in another clue).
'biting' indicates putting letters inside (some letters bite or swallow others).
'initially' says to take the initial letters.
'rolling over' is a reversal indicator.
The first letters of 'unfortunate child nipper' is 'ucn'.
'boil' enclosing 'ucn' is 'bucnoil'.
'bucnoil' backwards is 'LION-CUB'.
'in' acts as a link.
(Other definitions for lion cub that I've seen before include "Young mauler" , "Young big cat" , "Baby feline" , "beastly prince?" , "Creature" .)