Cowboy perhaps finding fish in two rivers (5)
I believe the answer is:
rider
'cowboy' is the definition.
(I've seen this in another clue)
'fish in two rivers' is the wordplay.
'two rivers' means two replacements for 'river'.
'fish' becomes 'ide' (type of fish).
'in' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'river' becomes 'r' (abbreviation for river).
'r'+'r'='rr'
'ide' inserted into 'rr' is 'RIDER'.
'perhaps finding' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for rider that I've seen before include "Comment from jury" , "Added proviso" , "Equestrian - clause" , "Horseman or addendum" , "Passenger" .)