Sale in pub invaded by extreme characters first of all (6)
I believe the answer is:
bazaar
'sale' is the definition.
(I know that bazaar is a type of sale)
'pub invaded by extreme characters first of all' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'invaded by' means one lot of letters goes inside another.
'extreme characters' becomes 'AZ' (letters at the extremes of the alphabet).
'first of all' becomes 'a' (1st letter of 'all').
'az'+'a'='aza'
'bar' going around 'aza' is 'BAZAAR'.
'in' acts as a link.
(Other definitions for bazaar that I've seen before include "Oriental market-place" , "Charity sale of many different things" , "An open market orient-style (6" , "Middle East market - fundraising event" , "foreign market" .)