How do you defend a number 10 who keeps drifting into wide areas?

I’d pass responsibility according to the space the 10 moves into. Centrally, your 6 may be the best player to screen or engage them. If the 10 drifts outside your winger, the full-back and winger may take more responsibility while the 6 protects the centre rather than following all the way across. The key is communication before the player receives. I’d give the unit a simple rule: do not follow a 10 into another teammate’s space if doing so opens a more dangerous central space behind you. Then train it with a free 10 who is encouraged to move anywhere between the lines. Your defenders have to keep passing the problem between midfielder, winger and full-back. There will be moments where the 6 follows wide. But the decision should come from whether somebody else can protect the middle, not from the number on the opponent’s shirt.