Common, half of ales in bar (9)
I believe the answer is:
prevalent
'common' is the definition.
(prevalent can mean common or widespread)
'half of ales in bar' is the wordplay.
'half of' indicates taking half.
'in' is an insertion indicator.
'bar' becomes 'prevent' (I've seen this before).
'ales' halved is 'al'.
'al' placed into 'prevent' is 'PREVALENT'.
(Other definitions for prevalent that I've seen before include "Commonplace" , "Ruling" , "Widely accepted" , "general" , "powerful" .)