River tours by arrangement (5)
I believe the answer is:
stour
'river' is the definition.
(I've seen this before)
'tours by arrangement' is the wordplay.
'by arrangement' is an anagram indicator.
'tours' with letters rearranged gives 'STOUR'.
(Other definitions for stour that I've seen before include "Harwich's river" , "English river" , "River Constable painted" , "Essex/Sussex border river" , "Constable country river" .)