I was under the impression from an earlier lesson that "propre" meant clean. So now it means "own" with no explanation provided.
When propre means clean it is placed after the noun and when it means own it is placed before the noun
ma propre voiture - my own car
ma voiture propre - my clean car.
now... could you say, "ma propre voiture propre" = my own clean car? :-)
If you want to translate a sentence by using dictionary definitions and say "well, it could be that", you need to stop and think for a second about exactly what "I have my clean reasons" means. The honest answer is it's not a viable translation. Very often you just have to go with the low-hanging fruit (the most obvious solution). Know that francophones also misunderstand one another by taking the wrong meaning of a word. The resulting confusion is a just reward.
Several French adjectives change meaning or gain distinction based on their relative location to the noun ( just whether they come before or after AFAIK). One I like: Je suis une femme simple pas une simple femme. I am an honest woman, not simply a woman.
I may have just guessed that "I have my own grapes. In my defense, I try to understand before they tell me unless I'm totally lost. I learn better that way.
And at least you didn't say you have clean grapes! :)
But you're right about the learning method. Struggle like hell to figure it out on your own, then check the correct meaning. The more you struggle, the more you learn!