Lukas Carey took three wickets for Glamorgan as the Welsh County reduced Derbyshire to 236/9 before bad light ended play early on day three.
Carey was the pick of the visitors' attack with 3-49, despite three dropped catches from his bowling.
The young seamer was ably supported by Marchant de Lange, Craig Meschede and Michael Hogan who all picked up two wickets as Glamorgan chipped away at the Derbyshire batting line-up.
"I thought we bowled quite well as a collective, we had a couple of dropped catches but a couple of quick wickets before the end of play has helped us," said Carey.
"The ball came out pretty decently for me but there's still things to work on, more consistency is the thing I've been working on. (Captain Michael Hogan) said if you worry about dropped chances, you're not going to concentrate on your bowling so try to forget them.
"The season has gone pretty well for me so far, so I'll try to finish off on a high. I've got my own targets, and it's always exciting to go out and impress people while we try to win these last few games."