Trendy drink reported in business — this can affect markets (7,7)
I believe the answer is:
insider trading
'this can affect markets' is the definition.
(insider trading affects the stock market)
'trendy drink reported in business' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'drink' becomes 'cider' (cider is a kind of drink).
'reported' indicates a 'sounds like' (homophone) clue.
'in business' becomes 'trading' (trading can mean operating as a business).
'cider' is a homophone of 'sider'.
'in'+'sider'+'trading'='INSIDER TRADING'
(Other definitions for insider trading that I've seen before include "Dodgy dealing" , "Illegal stock dealing" , "white-collar crime" , "Illegal stock exchange activity" .)