Poilievre is a lifelong politician who loves playing political games. This is just another one of those games. He is attacking Trudeau for not doing enough to prevent China from interfering with Canadian politics. Poilievre demanded an investigation into the matter, which was granted. Poilievre then immediately started calling the investigation fraudulent because the person in charge of it is biased. He has a history of doing this. The last time was when he demanded an inquiry into Trudeau's use of the Emergencies Act only to immediately claim the inquiry was biased and fraudulent when it found nothing wrong with Trudeau's actions.

In order to see the results of this investigation however, Poilievre needs security clearance, which he refuses to get. I doubt he has a secret he's trying to keep (although if he was denied security clearance that would be incredibly embarrassing). I think it's just another game. He can continue calling the investigation (that he demanded) as fake news and claim to be a victim of the evil Trudeau government.

Conservative articles defending Poilievre say that getting the clearance wouldn't change anything because it's not like Poilievre can share his findings after reading the report. These articles claim that if Poilievre gets cleared to read the report, then he now has to shut up about it, which hurts his campaign. I don't really think "Poilievre should only do things that help him get elected" is the greatest defence but it's the one I keep seeing.

He doesn't want one because then he can't complain about needing one.

The fact of a clearance has nothing to do with what Poilievre is doing. He is an anger clown who produces a non-stop stream of invective and complaints. Facts, relevance, and nuance are unwanted because they would interfere with the machine gun fire of grievances. So a clearance doesn't hold any value for him. He simply wants something to complain about.

Because if he gets it he will be given the inside scoop. But part of the deal with security clearance is that you are now in a circle of trust, and now you can't talk about whatever it is you now know about with people outside that circle.

That includes complaining about what's not happening because that might reveal info as well. That's the last thing he wants, since that's his entire modus operandi.

He is refusing it