at play—
1. playing games:
- It’s a bit like watching children at play, creating an imaginative world that they believe in completely.
2. engaged in gambling (playing cards, etc.):
- This gentleman has been at play at that there gaming-house over the way.
in play—
1. said of smth. done for amusement and not intended to be taken seriously:
- Don’t be offended; I only said it in play.
2. said of factors actively involved or having influence:
- Mighty forces are in play in the present round of negotiations.
Note: The expression does not correlate in meaning with the phrase in full play—in full action or operation; acting with full force:
- A desperate fight was going on … the rebel batteries were in full play.