Day with bird or mammal (8)
I believe the answer is:
mongoose
'mammal' is the definition.
(mongoose is a kind of mammal)
'day with bird' is the wordplay.
'day' becomes 'mon' (short for Monday).
'with' means one lot of letters go next to another.
'bird' becomes 'goose' (goose is a kind of bird).
'mon'+'goose'='MONGOOSE'
'or' is the link.
(Other definitions for mongoose that I've seen before include "Animal of the civet family" , "Animal that kills snakes" , "Carnivorous mammal that slays snakes" , "Predatory mammal known for killing snakes" , "Slayer of snakes" .)