Obscure relative by a river (7)
I believe the answer is:
unclear
'obscure' is the definition.
(I've seen this before)
'relative by a river' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'by' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation for river).
'uncle'+'a'+'r'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Not easy to grasp" , "Not apparent" , "Nebulous" , "Imprecise, hazy" , "An ulcer (anag)" .)