### Clue: Peg to go on endlessly about each rule, initially for person who's juvenile (8)
### Answer: TEENAGER 
 ### Explanation: Definition: person who's juvenile
 TEE(a peg;a short pin with a concave head used by golfers) + NAG(to go on endlessly about;to harry someone) + **1st letters, respectively, of**(…, initially) "each rule".
Heard this one?
Tiger Woods drives his BMW into a remote petrol station. As he bends to pick up the nozzle, two tees fell out of his pocket. The station attendant, ignorant of who Tiger is or even of golf, asks him what they are. "They're tees". "What are they for?" "They're for resting my balls when I'm driving." "Gee, those guys at BMW think of everything!"