Spray some PB on it.
Wait few mins/hour

this is what i used when i took my car apart for some rusted screws, best rust products out there.
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Zooka wrote Spray some PB on it.
Wait few mins/hour

this is what i used when i took my car apart for some rusted screws, best rust products out there.
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As he said, penetrating oil works wonders

If you were smart I would replace it instead of jimmy rigging it. you will regret it down the line when you muffler falls off. Don't be cheap.

Cut the clamp off

cut the clamp, they're not that expensive.

If after using penetrating fluid and it doesn't work you could also try heating the nut with a small torch (if its not near your gas tank). I usually just cut them off and use a 12gauge wire to hang the pipe while i plumb it in before tacking it all together then finishing the welds once i know everything is where I want it. If its sound your after its sometimes easier to replace a resonator with a glasspack (18", 22", 26" etc),
But thats my preference.
Might help to know what kind of vehicle it is?

As a mechanic I would personally cut the clamp off and replace it. They aren't expensive at all and will make the job a lot easier. If you don't have anything to cut it off, try going a socket size smaller and hammering the socket on. Then try to ratchet or impact it off. No matter what tho you should replace the clamp.