Island after island found in river (4)
I believe the answer is:
isis
'river' is the definition.
(I've seen this before)
'island after island' is the wordplay.
'island' becomes 'is'.
'after' says to put letters next to each other.
'island' becomes 'is'.
'is'+'is'='ISIS'
'found in' is the link.
(Other definitions for isis that I've seen before include "Revered figure" , "The goddess lives twice" , "Egyptian deity" , "Si, si - an Egyptian goddess" , "Si Si - she's an Egyption goddess" .)