New World rodent, plain, with tail (7,3)
I believe the answer is:
prairie dog
'new world rodent' is the definition.
(rodent native to North America)
'plain with tail' is the wordplay.
'plain' becomes 'prairie' (I've seen this before).
'with' says to put letters next to each other.
'tail' becomes 'dog' (synonyms).
'prairie'+'dog'='PRAIRIE-DOG'
(Other definitions for prairie dog that I've seen before include "North American burrowing mammal" , "Animal" , "Rodent" , "Underground community member" , "A ground squirrel" .)