DC 5V USB to replace AAA-battery source: down converter
In the end, while the LM317 worked to down-regulate the voltage to drive my AAA-pocket radio as I wanted, a 'quiescent current' was present while the speaker was switched off, so there goes that idea! Thanks for the tips, @BrianDrummond, @Passerby and @NicolasD.

While I've now resorted to rechargeable AAAs for the pocket radio instead, anyone else interested in down-regulating a 5V 1A USB source to drive a single AAA-battery powered device (like my pocket radio) could also look to the LM317.