On a rural road, a clearway sign indicates 'no stopping' (rather than 'no parking'). It an offence even to stop to pick someone up (other than at an authorised place, such as a layby). Just like speed limit signs, you're expected to notice them when you go past and then remember that the restriction applies until you're informed otherwise. (i.e. there are NO road markings!)
https://www.sabre-roads.org.uk/wiki/images/d/da/%22Clearway%22_sign%2C_Larne_-_Geograph_-_2012551.jpg
Just like speed limit signs though (other than 30 mph ones where there are streetlights at regular intervals), small repeater signs are placed periodically along your route as reminders that you're driving along a clearway. However the guidelines state that they only need to be 60 seconds apart when driving at the speed limit for that stretch of road. (Quote: "Where the speed limit is 30, 40 or 50 mph, the recommended spacing is 800m,1100m and 1350m respectively"). It's permitted to have them alternating between the left and right hand sides of the road though, so there might be a gap of up to 2700m (which is over one and a half miles) between those on your left, so it would be wrong to think of them as similar to parking restriction plates.
Clearways can also operate in urban areas. Just like bus lanes, the restrictions can apply for 24 hours or only for certain periods of the day:
https://www.drivejohnsons.co.uk/wp-content/uploads/2018/11/urban-clearway-start.jpg
Once again, there are NO road markings and the guidelines only require repeater signs every 400m, so you're expected to note and remember the initial signs as you pass them.
In most cases though, it's fairly obvious that the road that you're driving along is a main artery into a town or city, so it should be equally obvious that you can't park on an operational urban clearway.
The signs that you really need to be extra careful about are 'controlled zone' ones, such as this:
https://tinyurl.com/327xf6jk .
With such signs, you're once again expected to remember reading it, as there are NO road signs to indicate that you can't park (other than in authorised bays). Further, there are often NO repeater signs. You might, for example, encounter one as you drive towards a town centre and not see anything else about parking restrictions until you see a 'controlled zone ends' sign several miles after passing through the town centre. You're required to REMEMBER that you can't park on the roads (except where authorised to do so) anywhere at all between the two signs, including on all side roads. Many, many drivers get caught out through thinking that the absence of yellow lines (and/or restriction plates by the sides of roads) means that parking isn't restricted!