Sharp-witted little mammal departs (6)
I believe the answer is:
shrewd
'sharp-witted' is the definition.
(I've seen this before)
'little mammal departs' is the wordplay.
'little mammal' becomes 'shrew' (shrews are small mammals).
'departs' becomes 'd' (abbreviation eg in train timetables).
'shrew'+'d'='SHREWD'
(Other definitions for shrewd that I've seen before include "'Astute, savvy (6)'" , "Canny and clever" , "Wily" , "worldly wise" , "Penetrating" .)