Emperor discovering duck on north-eastern river (4)
I believe the answer is:
nero
'emperor' is the definition.
(Roman emperor)
'duck on north-eastern river' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'on' is a charade indicator (letters next to each other).
'north eastern' becomes 'NE' (abbreviation).
'river' becomes 'r' (abbreviation for river).
'ne'+'r'='ner'
'o' put after 'ner' is 'NERO'.
'discovering' acts as a link.
(Other definitions for nero that I've seen before include "Wicked fiddler" , "Fiddling Roman emperor" , "Emperor (who fiddled while Rome burned?) d. AD68" , "Tyrannical Roman emperor" , "He's reputed to have fiddled while Rome burned" .)