The phrase "a source of extra" is correct and usable in written English.
It can be used when referring to something that provides additional benefits, resources, or advantages beyond what is standard or expected.
Example: "The new marketing strategy proved to be a source of extra revenue for the company."
Alternatives: "an additional source of" or "a means of extra".
that gelatine was a source of extra energy, like Camel ci garettes.
Eating away from home can be a source of extra calories (due to large portions) as well as added sugar and sodium.
"The policemen hostile to me," wrote Daley in his memoir, published posthumously in 1987, "were ex-Guardsmen... who looked on homosexuals as a source of extra income.
And the debate has a dark side: allegations that some soccer federations exploit the matches, which don't count toward any league competitions, as a source of extra revenues.
It is, as Martin Wolf of the Financial Times puts it, both "a source of extra capacity" and a "source of demand".
Conversely, the addition of persulfate had a beneficial effect on the rate of degradation since it acted both as electron acceptor and a source of extra sulfate radicals.
Any of these side hustles can become a source of significant extra money.
Hydrocarbon impurities in the carbon black used in the metal binders, or impurities in the carbon blocks, could have provided a source of these extra Coulombs.
The rising cost of a new mortgage has already pushed down the number of people refinancing old mortgages, putting a crimp on a recent source of extra income for many households.
Such a system could potentially allow an iPhone, iPad or any portable device to build up a charge as a user moves around, giving up a continual source of extra juice that should at the very least ensure that you never run completely dry while on the go.
For Quintero, who was usually there alone during the week, the shooting range was a peaceful, somewhat solitary place — but a welcome source of extra income.
