Ground squirrel's plain tail (7,3)
I believe the answer is:
prairie dog
'ground squirrel's' is the definition.
(ground squirrel is another name for prairie dog)
'plain tail' is the wordplay.
'plain' becomes 'prairie' (type of plain).
'tail' becomes 'dog' (both can mean to follow).
'prairie'+'dog'='PRAIRIE-DOG'
(Other definitions for prairie dog that I've seen before include "Underground community member" , "North American burrowing mammal" , "A ground squirrel" , "Animal" , "Rodent" .)