Herdsman finding river in port (6)
I believe the answer is:
drover
'herdsman' is the definition.
(I know that drover is a type of rancher)
'river in port' is the wordplay.
'river' becomes 'r' (abbreviation).
'in' indicates putting letters inside.
'port' becomes 'dover' (English port).
'r' placed into 'dover' is 'DROVER'.
'finding' is the link.
(Other definitions for drover that I've seen before include "Person moving sheep or cattle from one place to another" , "Hugh the --, opera by Vaughan Williams" , "One driving cattle" , "I bring along many animals" , "Man who herds cattle" .)