I'm trying to insert a 6" section of 3/4" copper tubing in a heating
pipe that runs through a channel in my house slab. I have the zone
disabled, the water into the boiler turned off, the drain valve on that
zone open and a spigot used to release air and facilitate draining open.

But there is a constant drop of water in one side of the pipe that I
intend to solder. I know I can't solder a wet pipe. I'm reluctant to
try the old bread trick. Is there any kind of "solder", maybe epoxy
that will work with the wet pipe?

Suppose I fail in the repair attempt (this is the third time!). Is
there some sort of clamp that will seal off the leaking joint?

TIA.