Auction a small beer (4)
I believe the answer is:
sale
'auction' is the definition.
(I've seen this before)
'a small beer' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'ale' (I've seen this before**).
's'+'ale'='SALE'
(Other definitions for sale that I've seen before include "Period of reduced prices" , "See 12 Down" , "Auction - somewhere in Greater Manchester" , "Retail event - town in Greater Manchester" , "Period of low prices" .)