Bear, equid and big bird from the south (6)
I believe the answer is:
assume
'bear' is the definition.
(I know that bear can be written as assume)
'equid and big bird from the south' is the wordplay.
'equid' becomes 'ass' (ass is a kind of equid).
'and' means one lot of letters go next to another.
'big bird' becomes 'emu' (type of large bird).
'from the south' says the letters should be written backwards (in a down clue, letters go up).
'emu' backwards is 'ume'.
'ass'+'ume'='ASSUME'
(Other definitions for assume that I've seen before include "Suppose; put on" , "Accept as true without proof" , "Adopt or take for granted" , "Believe" , "Take as read" .)