OK. The Clark manual for the TK-60 shows usage with a 10channel receiver and skips the receiver channel 5 & 6 as it uses a FS-i6S TX.

Using an 8 channel receiver requires you all 8 channels used on the receiver.
These are the TK60 connections (if using 8 channel control, all 8 are filled)
CH1: Steering
CH2: Throttle
CH3: Elevation
CH4: Turret Rotation
CH5: MF Selection Switch
CH6: MF Execution Switch
CH7: Fire Loader's MG/NA
CH8: Fire Cannon/Fire Coax MG

On your TX you need to assign as follows for 8ch (remember that the Clark manual example is on a 10 channel RX & FS-i6S TX).
CH1: Steering (Stick)
CH2: Throttle (Stick)
CH3: Elevation (Stick)
CH4: Turret Rotation (Stick)
CH5: Switch SwB (3 position, indented on FS-i6S)
CH6: Switch SwC (3 position, indented on FS-i6S)
CH7: Switch VrA (3 position, Self-centering on FS-i6S)
CH8: Swicth VrB (3 position, Self-centering on FS-i6S)

For a 6 channel setup you only connect ch1-6 on both TK-60 and RX as it will sense connections on CH 7 & 8 (If attached to the RX)