### Clue: Right half player lost a litre of blood antigen (2,6)
### Answer: RH FACTOR 
 ### Explanation: Definition: blood antigen
  R(*right*) + HF("*half*" minus "a l"(*a litre*)) + ACTOR(*player* on stage).
Defn:  The critical *antigen* in *blood *which one either has or doesn't have.  Recorded in one's blood group as Rhesus positive or negative.