Hi Harry, welcome to the community! This idea has been mentioned before & I’m afraid it looks like Monzo won’t be adding this feature any time soon -
https://community.monzo.com/t/custom-categories-1/649/14
having said that, Monzo are going to review the categories soon so if you have particular categories that you’d like to suggest, please do list them 
My guess is sub-categories might solve your problem but apparently the team at Monzo is still considering several options.
In the meantime, since it’s probably unlikely that a category as specific as ‘cigarettes’ will be added, I’d recommend adding emoji - like 
- to these purchases instead, in case you’re interested, split categories have been suggested here. That would enable you to search for that emoji & see the total & average spent, combined with whichever other filters you apply. The caveat of course is that the totals will only work if you’ve only purchased the specific item that you want to track. This can still be useful though, I add a 
to my takeaways so that I can keep an eye on how many I buy each month..
Oh & Hugo (Monzo’s Head of Design) likes this idea too 