The stress and tension in your wheel is unbalanced. You might be lucky but its better to fix it.
You can replace a spoke without removing the cassette. Find a long spoke (eg from a 28" Chinese bike).
Cut off the elbow at the hub end.
Put a dog-leg bend at the hub end, ie sideways for about 3mm then down for about 8mm. The idea is to thread the dogleg through your spoke hole so the horizontal section acts as the elbow. Make sure you bend it at the correct length. You may need a couple of spokes to get it right. You may as well make a spare or 2 to carry.
Tighten the spoke to reach a compromise between even tension and true-ness (as measured at the brek blocks). You may need to wiggle and stretch any loseness from the spoke and re-tension.