Dog's bane (4)
'dog's bane' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flea that I've seen before include "Bug" , "Small parasitic insect" , "beastie?" , "little pest" , "In your ear, this insect is a rebuke" .)
(Other definitions for mange that I've seen before include "Disease affecting dog hair" , "Skin disease of cats and dogs" , "skin problem" , "Skin disease of mammals" , "Animal's skin disease causing loss of hair" .)