Transaction is small beer (4)
I believe the answer is:
sale
'transaction' is the definition.
(sales are a type of transaction)
'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 "Cut-price offer" , "Bargain time" , "Exchange of goods for fixed amount" , "Auction; Cheshire town" , "Bargain-price event" .)