Last in in the river (5)
I believe the answer is:
final
'last' is the definition.
(I've seen this before)
'in in the river' is the wordplay.
'in the' is an insertion indicator.
'river' becomes 'fal' (I've seen this before**).
'in' inserted inside 'fal' is 'FINAL'.
(Other definitions for final that I've seen before include "big match" , "Last match in a competition" , "Conclusive" , "binding" , "The last exam or leg of competition" .)