See one on European river (5)
I believe the answer is:
loire
'european river' is the definition.
(I've seen this before)
'see one on' is the wordplay.
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'one' becomes 'I' (Roman numeral).
'on' becomes 're' ('on' can be mean 'with reference to').
'lo'+'i'+'re'='LOIRE'
(Other definitions for loire that I've seen before include "Wine region" , "Longest French river" , "valley famous for its wines" , "Current French" , "River flowing through Tours and Nantes" .)