Hate delta on eastern river (6)
I believe the answer is:
detest
'hate' is the definition.
(to detest is to hate or loathe)
'delta on eastern river' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'on' says to put letters next to each other.
'eastern' becomes 'e'.
'river' becomes 'test' (the River Test).
'd'+'e'+'test'='DETEST'
(Other definitions for detest that I've seen before include "'Loathe, abhor (6)'" , "Cordially laothe" , "Deplore" , "Not like" , "Dislike heartily" .)