A number arranged to tour river in county (8)
I believe the answer is:
somerset
'county' is the definition.
(I've seen this before)
'a number arranged to tour river' is the wordplay.
'a number' becomes 'some' (I have seen 'Undefined number ' mean 'some' so perhaps 'number' could also mean 'some'**).
'arranged' becomes 'set' ('set' can be similar in meaning to 'arranged').
'to tour' means one lot of letters goes inside another (to tour can mean to go around).
'river' becomes 'r'.
'some'+'set'='someset'
'someset' placed around 'r' is 'SOMERSET'.
'in' acts as a link.
(Other definitions for somerset that I've seen before include "Maugham" , "English territory" , "head over heels" , "leap, occasionally" , "Southern English county" .)