Marketing event is small beer (4)
I believe the answer is:
sale
'marketing' is the definition.
(I know that sale is a type of marketing)
'small beer' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'beer' becomes 'ale' (type of beer).
's'+'ale'='SALE'
'event is' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sale that I've seen before include "Retail event - town in Greater Manchester" , "Clearance of goods at reduced prices" , "Bargain time" , "Offer period" , "Exchange of goods for fixed amount" .)