Reptile, one to the fore in S American region (6)
I believe the answer is:
iguana
'reptile' is the definition.
(iguana is a kind of reptile)
'one to the fore in s american region' is the wordplay.
I cannot really understand how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is located in the answer.
'american' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is found in the answer.
This explanation may well be incorrect...
(Other definitions for iguana that I've seen before include "Animal" , "Could be a monitor" , "Large crested lizard" , "Something like a monitor" , "American lizard with a spiny crested back" .)