Go by stream with a fierce guy (7)
I believe the answer is:
gorilla
'fierce guy' is the definition.
(informal term for a strong or aggressive man)
'go by stream with a' is the wordplay.
'by' says to put letters next to each other.
'stream' becomes 'rill' (rill is a kind of stream).
'with' is a charade indicator (letters next to each other).
'go'+'rill'+'a'='GORILLA'
(Other definitions for gorilla that I've seen before include "Aggressive-looking type" , "Heavy" , "brutal man" , "big mammal" , "Endangered anthropoid" .)