Ultimately then roid rage transformed American banker (3,6)
I believe the answer is:
rio grande
'american banker' is the definition.
'rio grande' can be an answer for 'banker' (I've seen this before). I am unsure of the 'american' bit.
'ultimately then roid rage transformed' is the wordplay.
'ultimately' indicates one should take the final letters.
'transformed' indicates an anagram.
The last letter of 'then' is 'n'.
'n'+'roid'+'rage'='nroidrage'
'nroidrage' with letters rearranged gives 'RIO GRANDE'.
Can you help me to learn more?
(Other definitions for rio grande that I've seen before include "Mexico-USA border" , "Waterway rising in Colorado" , "Colorado-to-Mexico river" , "River flowing between USA and Mexico" , "River boundary between the US and Mexico" .)