Best idea? Not quite but bordering (6)
I believe the answer is:
beside
'not quite but bordering' is the definition.
'beside' can be an answer for 'bordering'. I'm not sure about the 'not quite but' bit.
'best idea?' is the wordplay.
'best' becomes 'be' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'idea?' becomes 'side' (side is a kind of idea).
'be'+'side'='BESIDE'
(Other definitions for beside that I've seen before include "Next to - compared with" , "- - the point" , "Very near to" , "Neighbours" , "Next door to" .)