Not sure relative is on a river (7)
I believe the answer is:
unclear
'not sure' is the definition.
(unclear can mean uncertain or undecided)
'relative is on a river' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'is on' means one lot of letters go next to another (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation).
'uncle'+'a'+'r'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Doubtful" , "Not apparent" , "Not transparent or free from doubt" , "Not distinct in nuclear way" , "Imprecise, hazy" .)