Small beer in exchange for money (4)
I believe the answer is:
sale
'exchange for money' is the definition.
'small beer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'ale' (I've seen this before**).
's'+'ale'='SALE'
'in' is the link.
(Other definitions for sale that I've seen before include "Disposal - auction" , "From which one may get bargains" , "Bargain time" , "Auction; Cheshire town" , "Occasion for buying at reduced prices" .)