### Clue: Period in gym when core ignored, I bet a schoolboy error? (8,7)
### Answer: SPELLING MISTAKE 
 ### Explanation: Definition: a schoolboy error
 SPELL(a short period of time, as in "a cold spell") + IN + "gym" **minus** **its middle letter**(… when core ignored) + I + STAKE(to bet, as in "to stake your reputation on it").