Side where the brightest is best off (5)
I believe the answer is:
right
'side' is the definition.
'the brightest is best off' is the wordplay.
'off' is a deletion indicator (some letters taken off).
'brightest' with 'best' taken out is 'RIGHT'.
'where' acts as a link.
(Other definitions for right that I've seen before include "Complete" , "Not left" , "The Tory side" , "Punch" , "Fit" .)