Or in between? I have seen a job (university sector so gen quite good about flexible working etc) I would like to apply for but it's advertised as full time. I do 3 days a week. Would it be better to phone to discuss in advance of applying so that they can either do a flat no/ opt to try and match make a job share as part of application process, or to mention it if shortlisted or offered job? I want to be upfront but don't want to be ruled out on basis of hours before they've seen me/ my cv.
Be straight with them - at least you (& they) know from the off what hours you can work. Maybe send your CV then follow up with a phone call as soon as you can. As a recruiter I'd be annoyed if someone didn't mention they wanted PT when the ad said FT. They may consider it but you don't know till you ask.
My advice is always if you want to do significantly fewer than full time hours (and would therefore either require changes to the job or them to find you a job share partner), and would not be interested if this isn't possible then ask upfront.
If part time is either not a dealbreaker for you, or you only want something that is a very small change, for example slightly different start and finish times, or compressed hours, then wait until they are already keen on you.