have a fix

To acquire a required or necessary amount of something, especially that which one is addicted to or compulsively seeks. When I was a junkie, I would do just about anything in order to have a fix. I just can't get my day going until I have a fix of caffeine. I know she has to have a fix of nicotine because she's addicted, but it's going to kill her one day.

have a fix on (something)

To have a firm, clear understanding or determination of something. I hope everyone has a fix on the material, as you'll be tested on it next week. I thought I had a pretty good fix on English idioms, but I don't understand this one at all! My tutor was a huge help—I didn't remotely have a fix on polynomials until I met with her.