Although I have never encountered it personally, it sounds/looks like you have clematis borer, Alcathoe caudata. The sap visible is just the plant's natural sap juices - mold developing on that is not uncommon - but the maggots are probably the borer larvae, which feed on the roots and crown during the winter months.

Unfortunately, there's not a lot you can do to control borers short of some sort of systemic pesticide. And there may be too much damage done to the plant now for it to be very recoverable. I think replacement may be in order.

If you want to try to save that plant, then cut out the infected parts of the plant. Cut below that borer hole and then burn the top part of the plant. If the borer holes go down into the plant below the hole, then stick a wire into those holes to see if you can eliminate any larvae that may be in there. If that works then keep ahead of the problem by regularly pruning out the larger stems. The larger stems are preferred by borers. No guarantees, but it might work.

Steve