I'm a little confused as to the role that する is playing in phrases like はっきりとした and はっきりしている I know this is classed as a する verb, but it still doesn't make sense in the same way that nouns used with する do. How exactly do you translate these instances? Any answers appreciated, thanks.
The best translation of this usage of する would depend on the preceding adverb and the context.
With はっきり, it would be either "become" or "turn". The same goes for other adverbs such as しっとり、くっきり, etc. that, as はっきり, describe a state of a non-animate thing.
With adverbs like すっきり、どきどき、すかっと that describe a human sensation, "feel" would be the verb choice for translation.
BTW, you probably have a good book and/or teacher because you are using the word "adverb" correctly in discussing Japanese grammar. I say this because I have met so many Japanese learners who call words like はやく and たかく "adverbs" when those are merely conjugated adjectives.
Such words are not really adverbs, but mimetic words (
Some gitaigo may be used like nouns:
The topic of gitaigo words is generally kind of glanced over in many books but it is very important and can improve your understanding of the language quite a lot. There is a very nice post over at Nihon Shock describing it in more detail but I'll just mention that gitaigo words can take one of three forms:
They may be often written in katakana, especially for actual onomatopoeia (representing sound).
There's a nice gitaigo dictionary at NihongoResources.