Clue: Principal holding gym class in the Plough, say? (6)
Answer: HEPTAD
Explanation: Definition: the Plough, say
**HEAD **("principal") "holding" **P.T. **(physical training, so "gym class") 
*A heptad is a group of seven, and the Plough is a constellation with seven very bright stars, so is an example of a heptad.