The young midfielder broke the deadlock five minutes before the break, drilling home a low right-footed half-volley from the edge of the penalty area after Rovers had built down the right.
Clarkson struck the Portsmouth crossbar with a fierce 15th-minute shot from the corner of the penalty area.
Ryan Bird then squandered Pompey's best first-half chance, shooting tamely at goalkeeper Steve Mildenhall from 12 yards.
The visitors failed to create much after the interval and, despite the backing of more than 1,300 travelling fans, could have no complaints about the outcome,
And it took a stunning second-half save from Trevor Carson to keep out a Clarkson free-kick and prevent Rovers winning by a bigger margin.
- Sports & Recreation
- Oliver Norburn