One South American country ejecting unknown reptile (6)
I believe the answer is:
iguana
'reptile' is the definition.
(iguana is a kind of reptile)
'one south american country ejecting unknown' is the wordplay.
'one' becomes 'i' (Roman numeral).
'south american country' becomes 'guyana' (Guyana is an example).
'ejecting' suggests deleting specific letters.
'unknown' becomes 'y' (algebra).
'guyana' with 'y' removed is 'guana'.
'i'+'guana'='IGUANA'
(Other definitions for iguana that I've seen before include "Tropical American lizard" , "Animal" , "Lizard Darwin found in the Galapagos" , "Large crested lizard" , "Tropical spiny lizard" .)