Clarke has been sidelined for nearly two months after damaging his calf back in November, and City have been keen not to rush the forward back too soon to guard against the possibility of further injury.
The Irishman has featured in some warm-up work with the main squad in training this week before concentrating on his own conditioning programme with fitness coach Chris Short.
The aim now for the attacker, who hasn't featured since the 4-0 win over Rochdale on 12 November, is to try and play some part in Tuesday's Checkatrade Trophy round three tie at Cheltenham Town.
McCall said: "Billy feels good in himself at the moment and seems pain free. It is great to see him out running on the grass again and with a ball at his feet.
"He is obviously desperate to play as soon as possible but he has been out for eight weeks now.
"When you are coming back from that sort of period out, you need to work on your cardio and look after the other muscles as well.
"He is a very influential player for us, and it has clearly been frustrating for him and us to have him out for so long.
"He fits in with the way we like to play, and he is a good footballer that we have missed.
"With Fil (Morais), we knew he had a small tear (in his calf) so he is carrying on with his rehab work."
After missing last Saturday's victory over Chesterfield at the Northern Commercials Stadium, James Hanson will continue to be assessed in the build up to this weekend's trip to Shrewsbury Town.
McCall added: "He is still feeling some discomfort so we'll just see how he is over the course of the next few days.
"It shouldn't be anything too long for him but we will monitor his situation this week."