reducing voltage to power CREE LED headlamp
You can obtain a simple step-down switching regulator that can handle the required small dropout. If the 6V-HD battery puts out less than 4.5 V + V_dropout at some point in time you should consider using a buck-boost converter with either pure buck-boost or SEPIC topology.

The headlamp itself takes care of the LED driving which leads me to another solution. The headlamp probably does already include a constant current buck regulator and could be able to work directly with 6 V on the input, but this is not sure since 5 V is a common upper boundary for the input voltage. Only a risky experiment or a look at the used IC could reveal that.