Little island fellow is reptile (6)
I believe the answer is:
cayman
'is reptile' is the definition.
(I have seen 'Aquatic reptile ' mean 'cayman' so perhaps 'reptile' could also mean 'cayman')
'little island fellow' is the wordplay.
'little island' becomes 'cay' (I've seen this in another clue).
'fellow' becomes 'man' (fellow can mean a man).
'cay'+'man'='CAYMAN'
(Other definitions for cayman that I've seen before include "Type of crocodile and Caribbean islands" , "Fearsome creature" , "Alligator relative" , "Member of crocodile family" , "Type of 9 - islands" .)