Clear secretion?

When I went to ride Spree yesterday, her pasturemate Chance was back from the breeder. Despite being really crabby (hoo boy), Chance would also periodically secrete this squirt of clear liquid from her vagina. I don't know much about horses' cycles or breeding, so is this a liquid because she's in heat (like readying her to accept a stallion) or something indicating that she got knocked up?

From her vulva, yes very probably an indication that she is in heat.