Auction is small beer (4)
I believe the answer is:
sale
'auction' is the definition.
(type of sale)
'small beer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'ale' (type of beer).
's'+'ale'='SALE'
'is' acts as a link.
(Other definitions for sale that I've seen before include "Exchange of goods for fixed amount" , "Auction - somewhere in Greater Manchester" , "Period of lower prices" , "Offer period" , "Disposal - auction" .)