One country with no unknown lizard (6)
I believe the answer is:
iguana
'lizard' is the definition.
(I know that iguana is a type of iguanid)
'one country with no unknown' is the wordplay.
'one' becomes 'I' (Roman numeral).
'country with no unknown' becomes 'guana' (I am not sure about this - if you are sure you should believe this answer much more).
'i'+'guana'='IGUANA'
Can you help me to learn more?
(Other definitions for iguana that I've seen before include "Tropical tree-lizard" , "Creature" , "Could be a monitor" , "Lizard Darwin found in the Galapagos" , "American lizard with a spiny crested back" .)