Obscure relative seen by a river (7)
I believe the answer is:
unclear
'obscure' is the definition.
(unclear can mean obscure or vague)
'relative seen by a river' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'seen by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation).
'uncle'+'a'+'r'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Doubtful" , "hard to understand" , "Poorly described" , "Nebulous" , "Not transparent or intelligible" .)