Clue: Select the best poem before knight's engagement (8)
Answer: CULLODEN
Explanation: Definition: engagement
A charade of CULL ('select the best'; I would have defined culling as the selection of the worst for killing, but that means selection of the best for survival) plus ODE ('poem') plus N (chess notation, 'knight'), for the in 1746 which effectively ended the Jacobite rebellion (and the attempted restoration of the House of Stuart under Bonnie Prince Charllie) in favour of the House of Hanover.