Convert high voltage AC (10kV, 60Hz) to low voltage DC (5V)
Anything that requires connection to a 10kV AC line is potentially very, very lethal but, given that this CAN be done correctly and safely (i.e. employ someone trained to do so) then you can create a capacitor dropper that feeds a regular mains (110V or 230V) isolating transformer. I'm suggesting this because the power requirement is so small and using a 10kV primary transformer is going to be a b*tch of a lump in size!

Caution - make sure the capacitor dropper has very conservative ratings on the capacitors and include a fuse that can break the circuit should something go wrong at voltages up to 15kV. Use several capacitors in series to give a higher voltage rating and observe creapage distances on circuit boards (if used) and apply liberal air gaps.

In case you intend to try and rob an electricity company of a little power please justify your application.