Auction of small beer (4)
I believe the answer is:
sale
'auction' is the definition.
(I've seen this before)
'small beer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'ale' (type of beer).
's'+'ale'='SALE'
'of' is the link.
(Other definitions for sale that I've seen before include "Cut-price offer" , "Clearance of goods at reduced prices" , "See 12 Down" , "Exchange of goods for fixed amount" , "Disposal - auction" .)