So I’m digging a sort of retainage pond next to my old pond to accommodate the fish while I drain and clean out muck and silt, I have a small amount of water seeping in on the side towards my old pond is there anyway to stop the leak so that I can make sure it gets packed down good enough

IF you can stuff enough clay onto the leak, it could work.

I didn’t know if that would work, it’s just like a really slow seep just enough to keep the clay too wet to compact,

I have sealed horizontal water flows like that quite a few times without fail by digging down to the leak and below with a backhoe and fill it back in and compact it back with good clay and plenty of bentonite mixed in. Good luck