Check the battery voltage and than see if the same voltage is at the starter solenoid, If there is a voltage drop than clean all connections at the battery + / - cables and neg cable to frame. Also check the small terminal for full 12 / battery voltage when disconnected from solenoid when pushing starter button.
If the voltage and ground are good, the contacts in the starter solenoid may be worn, these contacts look like 2 small C shapes and contact a larger disc. When these wear down they cause this click condition, I have seen them be worn down and look like a knife blade, when new they are about 1/16 inch thick.
If they are worn you can disconnect the battery, pull the starter to be able to removes the contacts and plunger for replacement, these are the most common cause for this type / gear reduction starter, and you can get the parts at a starter and alternator rebuilder shop