"an indirect impact" is correct and usable in written English.
It is used to describe an influence or consequence of a certain action that is not direct or immediate. For example, "The increase in oil prices had an indirect impact on the inflation rate."
Taxing bad food choices at best can have only an indirect impact on health.
"We discovered that soy had an indirect impact on deforestation because of the land reselling effect," Weisser explains.
It also found an indirect impact from the messages: friends of friends were influenced as well.
"It would at the very least have an indirect impact on newspapers," said Elizabeth A. McNamara, a lawyer at Davis Wright Tremaine who often represents news media clients.
They also have an indirect impact via the wealth effect, as consumers increase their spending in response to improvement in their balance-sheets (see article).
They argue that reduced borrowing costs have at best an indirect impact on problems including the decline in government spending, concern about the European recession and the efforts of many consumers to pay their existing debts.
How much it charges one side of the market people who play games—has a knock-on effect on demand on the other people who develop them which has, in turn, an indirect impact on the first side.
A collapse in demand from Europe and America has had a direct impact on factories' orders, and an indirect impact on most other industrial areas, notably steel and construction.
Mobile also has an indirect impact on Alibaba's bottom line.
(This doesn't mean that Twitter's Instagram integration didn't have an indirect impact on the native app's popularity).
Lange had an indirect impact on what was later known as the revisionist debate within socialism.
