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The Lamia's wisdom drain is a supernatural "melee touch attack," hence it should follow the rules for any standard attack action, and it does not activate simply whenever the creature touches something.
The Cleric touch ability is a Spell-like ability, and follows the rules for those (p. 221 CRB), essentially acts as a spell with casting time of one action.
I would rule that the Lamia could use the Wisdom Drain when making a touch attack with a withheld charge, but could not use the Cleric touch ability.