Remain working with river sailor (7)
I believe the answer is:
mariner
'sailor' is the definition.
(mariner means a sailor)
'remain working with river' is the wordplay.
'working with' is an anagram indicator (working with the letters).
'river' becomes 'r' (abbreviation for river).
'remain' anagrammed gives 'marine'.
'marine'+'r'='MARINER'
(Other definitions for mariner that I've seen before include "Person sailing" , "Bluejacket" , "Sea dog" , "Jack" , "Seafarer, old salt" .)