Outcast one left in Devon river (5)
I believe the answer is:
exile
'outcast' is the definition.
(I've seen this in another clue)
'one left in devon river' is the wordplay.
'one' becomes 'i' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'devon river' becomes 'exe' (I've seen this before).
'i'+'l'='il'
'il' going inside 'exe' is 'EXILE'.
(Other definitions for exile that I've seen before include "One expelled (from their native country)" , "He is forced to live in another country" , "Deport" , "Person forced to live abroad" , "Refugee" .)