Short day with bird or mammal (8)
I believe the answer is:
mongoose
'mammal' is the definition.
(mongoose is a kind of mammal)
'short day with bird' is the wordplay.
'short day' becomes 'mon' (short for Monday; 'short' suggests a short form).
'with' is a charade indicator (letters next to each other).
'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 "Snake killer" , "Carnivorous mammal that slays snakes" , "Predatory mammal known for killing snakes" , "Cat-like creature with long body and bushy tail" , "Agile cat-like creature with long body and tail" .)