Organize a row across river (7)
I believe the answer is:
arrange
'organize' is the definition.
(arranging is a kind of organizing)
'a row across river' is the wordplay.
'row' becomes 'range' (I've seen this before**).
'across' means one lot of letters goes inside another.
'river' becomes 'r'.
'range' enclosing 'r' is 'rrange'.
'a'+'rrange'='ARRANGE'
(Other definitions for arrange that I've seen before include "Place in order" , "Set in place" , "Bring about or put in order" , "Organise, plan" , "Put in sequence" .)