Alex Hughes struck his first century of the summer, but Glamorgan fought back to close with a lead of 122 runs heading into the final day at Swansea.
Hughes’ fine knock of 103 helped Derbyshire to 362 all out, but the hosts lost five wickets for as many runs to post a lead of 79 runs, less than they would have been targeting at 357-5.
Hamidullah Qadri claimed two Glamorgan wickets, but Usman Khawaja and Kiran Carlson added an unbeaten 153 runs for the fifth wicket to give Glamorgan a sizeable overnight lead.
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