Rodent with plain tail (7,3)
I believe the answer is:
prairie dog
'rodent' is the definition.
(prairie dog is a kind of rodent)
'with plain tail' is the wordplay.
'with' means one lot of letters go next to another.
'plain' becomes 'prairie' (type of plain).
'tail' becomes 'dog' (both can mean to follow).
'prairie' next to 'dog' is 'PRAIRIE-DOG'.
(Other definitions for prairie dog that I've seen before include "Rodent" , "A ground squirrel" , "Underground community member" , "North American burrowing mammal" , "Animal" .)