Relative area reduced by half? Not sure (7)
I believe the answer is:
unclear
'not sure' is the definition.
(unclear can mean uncertain or undecided)
'relative area reduced by half?' is the wordplay.
'relative' becomes 'uncle' (uncle is a kind of relative).
'reduced by half?' indicates taking half.
'area' halved is 'ar'.
'uncle'+'ar'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Imprecise, hazy" , "Not transparent or free from doubt" , "Not distinct in nuclear way" , "hard to understand" , "Poorly described" .)