Example 1
Burt may have been the star of the JV team, but on the varsity squad, he just can't cut the mustard.
Sadly, we're not talking about a bologna sandwich-making team. Can't cut the mustard is an idiom that refers to someone who isn't adequate enough to compete or participate.
Example 2
The thieves took the couch, the coffee table, the TV, the fake ficus… the whole nine yards.
Who steals a plastic tree? We don't know, but we do know that the whole nine yards is an idiom that means everything or all of it.
Example 3
After Tiffany told Darren about Sharla's massive crush on him, Sharla had an axe to grind with her supposed best friend.
While we'd like to tell you that axe grinding is an ancient bonding ritual between BFFs, it's actually an idiom that means to have a dispute with someone. Watch out, Tiff.