Vixen showing cunning endlessly (5)
I believe the answer is:
shrew
'vixen' is the definition.
(I've seen this before)
'cunning endlessly' is the wordplay.
'cunning' becomes 'shrewd' (I've seen this before).
'endlessly' means to remove the last letter.
'shrewd' with its final letter taken off is 'SHREW'.
'showing' is the link.
(Other definitions for shrew that I've seen before include "Small mouselike animal" , "Long-nosed mammal" , "Scold, termagant" , "insect eater" , "Small mammal with a long snout" .)