Medical instrument outstanding policeman found in river (10)
I believe the answer is:
'medical instrument' is the definition.
(I know that rhinoscope is a type of medical instrument)
'outstanding policeman found in river' is the wordplay.
'outstanding' becomes 'os' (accounting abbreviation).
'policeman' becomes 'cop' (slang term).
'found in' means one lot of letters goes inside another.
'river' becomes 'rhine' (Rhine River is an example).
'oscop' put into 'rhine' is 'RHINOSCOPE'.