Crafty little mammal departs (6)
I believe the answer is:
shrewd
'crafty' 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' (common abbreviation e.g. train timetables).
'shrew'+'d'='SHREWD'
(Other definitions for shrewd that I've seen before include "Sharp-witted" , "Penetrating" , "keen" , "Cunning" , "Astute, calculating" .)