Just Give it to Him
As the GM, you don't need a specific rule to justify it. The NPC is a villain you created for story-purposes, there is no reason to restrain yourself to the rules used for creating PC characters. Especially since this is going to be a one-off usage, it isn't worth going through the mechanics to justify it.
GMs vary in how much they stick to the rules. In this case, there are no rules constraining your choice. The spell lists and class features are rules for creating player characters. They should not be constraints (but possibly rough guidelines) for creating challenges.
Defense Against the Rules Lawyer
Rules lawyers present a different challenge. An astute player might wonder how it's even possible for a cleric to be using a psionic ability, given that they don't have any levels in a psionic class.
Don't get sucked in to the lawyering. The abilities in player class descriptions are not the sum of everything possible in the game universe - just the options that players are allowed to choose from. There's a reason it's called the "Player's" handbook.
A second defense against the rules lawyer is to remind them that you are the judge. The GM is tasked with creating the game world and deciding what is reasonable. Basically, you are the authority - not the rule book. Of course, if you are going to deviate from the core rules significantly it is good etiquette to let everyone know ahead of time.
While using class mechanics to justify an ability isn't that important, an in-game explanation is. Did the cleric receive the ability as a boon from an evil deity or supernatural creature? Maybe it comes from an artifact he retrieved from the ruins of an ancient psychic civilization. If your storytelling-reason is compelling, players won't need to look under the hood.