French plane, one ditched in river (4)
I believe the answer is:
avon
'river' is the definition.
(I know that Avon is a river)
'french plane one ditched' is the wordplay.
'french plane' becomes 'avion' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'ditched' is a deletion indicator.
'avion' with 'i' taken out is 'AVON'.
'in' is the link.
(Other definitions for avon that I've seen before include "Name of several English rivers" , "English river flowing through Stratford" , "English river, flows through Stratford" , "Name of three English rivers" , "Shakespeare its swan" .)