A fellow taking in Australia going up river (6)
I believe the answer is:
amazon
'river' is the definition.
(Amazon is an example)
'a fellow taking in australia going up' is the wordplay.
'fellow' becomes 'man' (I've seen this before**).
'taking in' is an insertion indicator.
'australia' becomes 'oz'.
'going up' is a reversal indicator.
'oz' back-to-front is 'zo'.
'man' going around 'zo' is 'mazon'.
'a'+'mazon'='AMAZON'
(Other definitions for amazon that I've seen before include "Fighting woman from a great river" , "Female warrior of Greek myth" , "World's largest river" , "One of a mythic nation of women warriors" , "big river" .)