Verb. 1. To notice, see or watch.
e.g."Once I clocked him looking suspicious, he left the shop without stealing anything."
2. To hit or punch, mainly on the head
e.g."I'm going to end up clocking that idiot if he doesn't shut his big mouth!"
3. Noun. The face.
Clock has been used to great effect in TV cop shows as The Sweeney (which in itself is rhyming slang – Sweeney Todd = Flying Squad!) and The Professionals. It also crops quite a lot in Eastenders and the films of the Crown Prince of Mockney, Guy Ritchie.
It appears to originate in London but also turns up in Australian slang dictionaries