Making A USB Charge Cable (to replace 2.4V power supply) - Buck Converter?
Something like this LM2596S based DC-DC convertor would work really well. I have had good experience buying on eBay. Then you need a micro-B USB receptacle and a DC power plug but you have to check the dimension of the hair clipper power plug. Plug the receptacle into whatever USB charger you already have to check the voltage. Wire up the convertor and adjust it to get the output voltage matching the clipper power adapter you are replacing. Then wire up the power plug and try it out. If the USB charger doesn't have enough power it won't work but it won't catch on fire. In that case you will have to upgrade the USB charger, but that will be good for charging your other stuff too.