Mouse-like creature, wily, no end (5)
I believe the answer is:
shrew
'mouse-like creature' is the definition.
(I've seen this before)
'wily no end' is the wordplay.
'wily' becomes 'shrewd' (I've seen this before).
'no end' means to remove the last letter.
'shrewd' with its final letter taken away is 'SHREW'.
(Other definitions for shrew that I've seen before include "Small mouselike mammal" , "Petruchio tamed one" , "Tamable woman?" , "Little mammal; a scold" , "Mouse-like animal or scolding woman" .)