I can't get my clock on my Apple Watch to stay on all the time

What Apple watch do you have? It is only supported on 5 or higher.

If your watch supports it, you can set it by going to Settings on the watch, then Display & Brightness, then scroll down to Always On to check value. Settings is the gear icon when you click the crown to show the app icons.

Info on setting it can be viewed here: https://support.apple.com/en-us/HT210428

I have a 6 , and it has only suddenly started to do this , have turn off then redid the always on in settings but it still goes blank

Check to see if you have theatre mode turned on and turn it off;