A wild gorilla eats head of tiny reptile (9)
I believe the answer is:
alligator
'reptile' is the definition.
(alligator is a kind of reptile)
I cannot understand how the rest of the clue works.
(Other definitions for alligator that I've seen before include "What can go with pear" , "predator" , "Cold-blooded killer, maybe" , "From fearsome beast" , "one bites voraciously?" .)