One of the questions CC ask for their TC is "tell us about your extra-curricular, positions of responsibility and an achievement of importance you are most proud of". The word limit is 400 words, and I can't seem to meet that, I get stuck at about 300 before I begin to ramble and add nothing to the substance of my application. Is it still worth submitting or to keep brainstorming to meet/get closer to the word limit?
Also, is it acceptable to put ranking first in my cohort for a module as my achievement? I imagine academics aren't really what they are looking for in this answer, but wasn't too sure.
Just because the word limit is 400 doesn't mean you have to write to the full word limit to have a good answer and you shouldn't base whether your answer is worth submitting or not based on the word limit. It's more about quality than quantity here. If I was writing an answer to this question, I would ask myself if there is anything more about the experiences I've put down that I can speak about, or if there's any more experience I can speak on. I'm not sure if you have done this, but I would also put down all of my extracurriculars, positions of responsibilities and achievements of importance down on a list, which should help you think about what to write.
I would also definitely get someone else to read it, like a friend, who might be able to suggest what more you should talk about. I also recommending using the TCLA application review service, as they can provide you in-depth feedback on your answer. If after all that, you believe you have nothing more substantial to add and that your answer is good, then by all means submit it.
As for your ranking in a module, this could be an achievement worth mentioning, especially if you've got an award for ranking first. Of course, this depends on if in other areas of the application they have given you the opportunity to discuss academic awards. If not, then I would talk about it here briefly as part of my answer.
Hope this helps!