Small mammal; sneaky person (6)
I believe the answer is:
weasel
I believe this clue is a double definition.
'small mammal' is the first definition.
(I've seen this before)
'sneaky person' is the second definition.
(I've seen this in another clue)
(Other definitions for weasel that I've seen before include "Sneaky person - small, long-necked carnivorous animal" , "Rat" , "Double-dealer" , "One deceiving" , "Musteline animal" .)