Essex were made to do without Monty Panesar who was dropped for
disciplinary reasons but coped admirably in his absence as Reece Topley took six wickets to help bowl Glamorgan out for 244.
Panesar, left out as a result of an issue with timekeeping, was replaced by leg spinner Tom Craddock, although he will be available for Friday's T20 game on Friday.
Craddock proved an expensive replacement with his seven overs costing 30 but he did pick up the wicket of Will Bragg, who top-scored for Leicestershire with 93 as he continued his excellent form.
But it was Topley, making his first Championship outing of the season after a stress fracture of his back during the winter, who did the damage as Glamorgan only managed one more half-century.
Topley yorked Tom Lancefield in only the second over of the day after the visitors elected to bat. He struck again just before lunch, having Ben Wright caught at gully, before returning with the new ball after tea to claim four wickets in five overs, including two in successive balls, to polish off the innings.
Glamorgan looked like they had recovered from a shaky start which saw them reduced to 112 for 3 just after lunch as Bragg passed 500 championship runs for the season in putting on 68 with Chris Cooke for the fourth wicket.
Bragg's departure in the penultimate over before tea was a harbinger of doom for Glamorgan who returned after the interval to lose their last six wickets for the addition of just 53 runs. Cooke was last man out, edging Topley to slip, in the final over of the day.