Voyager coming upon river on top edge of rainforest (10)
I believe the answer is:
adventurer
'voyager' is the definition.
(thesaurus)
'coming upon river on top edge of rainforest' is the wordplay.
'coming' becomes 'advent' (advent is the arrival or coming of something).
'upon' means one lot of letters go next to another.
'river on top edge of rainforest' becomes 'urer' (I can't justify this - if you can you should give a lot more credence to this answer).
'advent'+'urer'='ADVENTURER'
Can you help me to learn more?
(Other definitions for adventurer that I've seen before include "One seeking exciting experiences" , "An opportunist" , "hero?" , "One loving dangerous exploits" , "Explorer" .)