Not easy to make out relative by a river (7)
I believe the answer is:
'not easy to make out' is the definition.
(something unclear is hard to make out)
'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).
(Other definitions for unclear that I've seen before include "Ambiguous" , "Not apparent" , "Not certain" , "Indistinct" , "Not definite" .)