you must solder the throttle cable into the clevis, but before you do it make sure that the nyrod is not restricting the movement, its a Bit_h to shorten it when the clevis is soldered.
soldering the servo clevis [:-],
well for me its not the preferred connection because i like to have this degree of freedom otherwise thats it and you cannot change anymore.
Lock tight doesnt do it? get 2-56 nuts you will need them in the future.
meanwhile i would go with some nylon clevis instead of soldering them permanently.

Alex