Jonny Wilkinson - who scored all Toulon’s points - thought he had earned a draw with his 79th-minute drop goal, but Australian fly-half James capitalised on an infringement by compatriot Matt Giteau to seal the win.
Napolioni Nalaga and Regan King had scored early tries for the hosts, before the boot of Wilkinson brought the visitors level. The win sees Clermont close the gap on Toulon to just two points.
Racing Metro slumped to a surprise 16-17 home defeat to basement boys Mont de Marsan.
England international Olly Barkley, playing fly-half, scored and converted his first Racing try before kicking three penalties, but Simon Lucu’s try and an Antoine Vignau-Tuquet drop goal saw the visitors fight back to victory.
It was not all bad news for Paris though as Stade Francais beat Agen 20-13. Julien Arias touched down for the Parisians, with full-back Jerome Porical kicking five penalties.
Many Top 14 stars were missing on international duty, but France's Dmitri Yachvili stayed behind as part of his recuperation from injury – he kicked five penalties as Biarritz beat Perpignan 15-3.
Other results saw Castres win 16-13 at Bordeaux, while Grenoble beat Bayonne 9-6.