It’s not obvious. On an iPhone 16, if you click on Search, type in the word “anorak”, you should see “anorak” and “anoraks” appear. Click on “anorak”.
At the bottom of the screen, the middle icon is a hollow box with an arrow pointing upwards. Click that, and it should show the fifth option as “Full text search”. Click that, and in the search box at the top type “anorak”.
Then click “Search” and it should run through 52121 definitions and find 3 matches, including “cagoule” and “wind”.
If that doesn’t happen I would suggest deleting and reinstalling the app.