The act of drinking 3 pints of beer (lager, bitter, stout etc) without going to the toilet until the 3rd pint is finished. (1 UK pint = 568.2ml).
In the pub, sink 3rd full pint, desperate for a piss, stand up to go and announce "I've just done a John Craven".
Keep it under your hat until complete - your drinking companions will be green with envy.
No idea where this definition comes from, but John Craven was a popular children's television news presenter on the BBC in the UK 1972-1989.