Mean to be bottom (4)
I believe the answer is:
base
I believe this is a double definition.
'mean' is the first definition.
('base' can be a synonym of 'mean')
'bottom' is the second definition.
(base is a kind of bottom)
'to be' is the link.
(Other definitions for base that I've seen before include "Meanspirited" , "Reprobate" , "Pedestal" , "Interest charged by the Bank of England to commercial banks" , "Mil. operations centre" .)