Unknown character reported in Devon river (3)
I believe the answer is:
exe
'devon river' is the definition.
(I've seen this before)
'unknown character reported' is the wordplay.
'unknown character' becomes 'x' (X can represent any person, as in Mr X etc).
'reported' indicates a 'sounds like' (homophone) clue.
'x' sounds like 'EXE'.
'in' acts as a link.
(Other definitions for exe that I've seen before include "River of southwest England" , "source of water" , "River in SW UK" , "Topsham's river" , "Devon river" .)