Tower Controller in training issued a 'cancel takeoff clearance' to an air carrier departure when noting a previous cleared runway crossing aircraft still on the runway.

Narrative

A Gulfstream was given the traffic in position and told to cross Runway XXL and taxi into parking. Air Carrier X was told to line up and wait on XXL. Aircraft Y asked me to verify the landing clearance on XXL. I told aircraft Y; 'Runway XXR clear to land.' There was no response from Aircraft Y. I asked aircraft Y to verify Runway XXR; the right side; clear to land. Aircraft Y said he needed XXL to land. I scanned the runway and saw that the Gulfstream was halfway across the runway. Using anticipated separation; I cleared Air Carrier X for takeoff. I scanned the runway again and noticed that the Gulfstream was still on the runway and moving slowly. I told Air Carrier X to stop the roll. My trainer reiterated; 'Air Carrier X hold your position; cancel takeoff clearance.' Air Carrier X acknowledged cancel takeoff roll. I sent aircraft Y around. Air Carrier X requested a place to park for brake cooling numbers. I feel that often times in training; there has to be a certain traffic count for Management to feel that training is productive. However; this puts the trainee in a position where he/she may not have worked the position in several days. I feel that continuous training; regardless of traffic count; is good for the trainee to keep them in the groove of working that position.