A river flowed in Scottish isle (5)
I believe the answer is:
arran
'scottish isle' is the definition.
(island off Scottish coast)
'a river flowed in' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'flowed' becomes 'ran' ('run' can be a synonym of 'flow').
'in' is an insertion indicator.
'r' placed inside 'ran' is 'rran'.
'a'+'rran'='ARRAN'
(Other definitions for arran that I've seen before include "Scottish island" , "Firth of Clyde island" , "Scottish territory" , "Isle near Firth of Clyde" .)