Clue: 62.5% turn back on time as Queen (5)
Answer: REIGN
Explanation: Definition: time as Queen
The first five of the 8 letters ('62.5%') of REIGN[ore] ('turn back on'). I am not convinced that I have it right, but I cannot see anything better. *Thanks Flavia @8: it is REIGN[ite] – which I looked at and rejected – a clear case of tunnel vision, when I could not get past the other meaning of 'turn back on'.