LDRs are optically-coupled switches - it stands for Light Dependent Resistor, although really this is only half the switch, the other half is an LED (Light Emitting Diode) inside the same encapsulated unit.
The Rectoverb is a completely different amp though - there are some different tubes used for each channel, and the switching is done with relays not LDRs, so it won't be that!
First, try changing preamp tube V2 (second from the input jack, next to the one under the metal cover) - V2A is used on the clean channel only. If you don't have a spare 12AX7, try temporarily swapping V2 for V3 - if the fault moves to the lead channel, it's the tube.
If it's not that it's likely to be a relay or some other issue as there are no other tubes that only affect the clean channel.