One expressing disapproval outside unknown pub? (6)
I believe the answer is:
boozer
'pub?' is the definition.
(boozer can informally mean a pub)
'one expressing disapproval outside unknown' is the wordplay.
'one expressing disapproval' becomes 'booer' (someone booing is expressing disapproval).
'outside' is an insertion indicator.
'unknown' becomes 'z' (algebra).
'booer' enclosing 'z' is 'BOOZER'.
(Other definitions for boozer that I've seen before include "Heavy drinker" , "Inn" , "Pub (sl.)" , "Pub (slang)" .)