Roman Number Wordplay/9 from 6, 10 from 9, 50 from 40

Puzzle

 * If from six ye take nine, 'and from nine ye take ten
 * (Ye youths, now the mystery explain),
 * And if fifty from forty be taken, there then,
 * Shall just half a dozen remain.

Solution
Putting them together gives:
 * $\mathtt{SIX}$