Exclude lower classes at pub (5)
I believe the answer is:
debar
'exclude' is the definition.
('debar' can be a synonym of 'exclude')
'lower classes at pub' is the wordplay.
'lower classes' becomes 'de' (I've seen this in other clues).
'at' says to put letters next to each other.
'pub' becomes 'bar' (I've seen this before).
'de'+'bar'='DEBAR'
(Other definitions for debar that I've seen before include "Rule out" , "Prevent from doing something" , "Black" , "Shut out, preclude" , "Exclude from a place" .)