The word "fat" can be an adjective (1) or a noun (2):
- (1) The fat man; or
- (2) The man liked to eat fat.
If I make (1) "fatter", this is an example of inflection (same word class).
Yes, that's correct.
In contrast, while not a grammatical sentence if you make (2) "fatter", this would be an example of derivation due to a change in word class.
Am I on the right track with my understanding?
I don't understand how you can convert the noun "fat" to "fatter." An example of derivation would be the verb "fatten."