Spiritual Meaning:
Today sport is often used as an energy release, so we are more likely to dream of a sports arena or football pitch rather than an ancient arena.  Spiritually an arena suggests a ritualized form of combat and sometimes sacrifice, but can also suggest the need for space in which to expand or enjoy group activities.

Psychological / Emotional Perspective:
We are developing a new focus of attention or an area of conflict. This conflict may need to be brought out into the open for due consideration.

Everyday Material Aspects:
Dreaming of being in an arena, either as a player or as a spectator, highlights the fact that we may need to make the decision to move into a specifically created environment, one which gives more room for self-expression and creativity or theatricality.

You might like to look at the entries for Competition, Games and Rite / Ritual.