Adopt a son, setting up fast birds from down under (6)
I believe the answer is:
assume
'adopt' is the definition.
(synonyms)
'a son setting up fast birds from down under' is the wordplay.
'son' becomes 's'.
'setting up' says the letters should be written backwards.
'fast birds from down under' becomes 'emus' (Australian birds known for their running speed).
'emus' reversed gives 'sume'.
'a'+'s'+'sume'='ASSUME'
(Other definitions for assume that I've seen before include "Believe" , "Suppose; put on" , "Take for granted, suppose" , "Adopt or take for granted" , "Take to be the case" .)