Kid is initially bitten by reptile, commonly a fierce fighter (9)
I believe the answer is:
gladiator
'fighter' is the definition.
(I know this)
'kid is initially bitten by reptile' is the wordplay.
'kid' becomes 'lad' (lad can mean a boy or young man).
'initially' says to take the initial letters.
'bitten by' indicates putting letters inside (I've seen this in other clues).
'reptile' becomes 'gator' (I have seen 'Short reptile' mean 'gator' so perhaps 'reptile' could also mean 'gator').
The first letter of 'is' is 'i'.
'lad'+'i'='ladi'
'ladi' placed within 'gator' is 'GLADIATOR'.
'commonly a fierce' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for gladiator that I've seen before include "Man appearing in ring" , "Circus performer" , "Roman arena fighter" , "Spartacus, for example" , "Warrior" .)