Discover American fellow leaving for river (5)
I believe the answer is:
indus
'for river' is the definition.
(Indus is an example)
'discover american' is the wordplay.
'discover' becomes 'ind' (I can't explain this - if you can you should give a lot more credence to this answer).
'american' becomes 'us'.
'ind'+'us'='INDUS'
'fellow leaving' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for indus that I've seen before include "River flowing from Tibet to the Arabian Sea" , "Sacred flower" , "9 of Pakistan" , "Major river of Pakistan" , "River of India and Pakistan" .)