Vixen showing endless cunning (5)
I believe the answer is:
shrew
'vixen' is the definition.
(I've seen this before)
'endless cunning' is the wordplay.
'endless' means to remove the last letter.
'cunning' becomes 'shrewd' (I've seen this before).
'shrewd' with its last letter taken away is 'SHREW'.
'showing' acts as a link.
(Other definitions for shrew that I've seen before include "Insectivorous mammal" , "Mouselike mammal with long snout" , "Vixen" , "Virago" , "Little mammal; a scold" .)