why would a digital clock in car not come back on when battery leads have been disconnected to charge battery?

Probably because when you re-charged the battery, it caused a power surge which in turn might have burned out the fuses which the clock and lighter work off of. Find the fuse box. Your service manual shows exactly where it is. Usually in a prominent place under the dash or under the hood. It's a plastic box. Open it. The box containes all the fuses for the car and also contains a list of what each fuse is for. Find the clock and lighter fuse. Replace them with exact replacement. You can purchase them in any auto parts store. Bring the old fuses with you so you will get an exact replacement. Done.