The Colorado Rapids star has been with the Royals for the past few weeks and will now remain in Berkshire in a bid to win a contract this winter.
Clarke said: "Dillon has come over from the MLS and is working with us. He will stay with us for another week.
"The MLS is out of season as his team didn't qualify for the play-offs. So he's working with the rest of the group and I know he's enjoying it.
"But it will be a decision for January. It can't be done until then so we'll continue to look at Dillon in training. So far he's done well. He's a talented midfielder."