Manufacturer is German on River Test (13)
I believe the answer is:
industrialist
'manufacturer' is the definition.
(I've seen this before)
'river test' is the wordplay.
'river' becomes 'indus' (Indian river).
'test' becomes 'trialist' (I am not sure about this - if you are sure you should believe this answer much more).
'indus'+'trialist'='INDUSTRIALIST'
'is german on' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for industrialist that I've seen before include "Manufacturer" , "Ford, perhaps" , "Businessman - I trail nudists (anag)" , "Businessman - distrust in Ali (anag)" , "Manufacturing chief" .)