Lower barrier enclosing empty space (5)
I believe the answer is:
baser
'lower' is the definition.
('base' can be a synonym of 'low')
'barrier enclosing empty space' is the wordplay.
'barrier' becomes 'bar' (bar is a kind of barrier).
'enclosing' is an insertion indicator.
'empty' says to hollow out the word (remove centre letters).
'space' with its middle removed is 'se'.
'bar' going around 'se' is 'BASER'.
(Other definitions for baser that I've seen before include "meaner" , "less" , "Not so admirable" .)