Eustace, a former Rams loanee, is a free agent after his contract with Watford expired.
The Hornets wanted the 33-year-old to remain at Vicarage Road in a coaching role but he is not ready to hang up his boots and has offers from other clubs on the table.
Clough told the Derby Telegraph: "John is training with us. He has a couple of playing offers from clubs but we have said 'come and train with us for a couple of weeks and see how it goes'."
Eustace spent the final two months of the 2008/09 season on loan at Pride Park from Watford.