Not bottom (3)
I believe the answer is:
top
'not bottom' is the definition.
('top' is the opposite to 'bottom')
This is all the clue.
(Other definitions for top that I've seen before include "SECOND PART OF 10 ACROSS" , "Summit, peak" , "Peak; child's toy" , "Acme or apex" , "cap and > sweater" .)