### Clue: Monarch possibly tracked horse at Barnet's border (8)
### Answer: HAIRLINE 
 ### Explanation: Definition: Barnet's border
  AIRLINE(an example of which is,possibly British charter and scheduled airline, Monarch, based at Luton Airport) **placed after**(tracked,followed) H(slang for heroin, another slang for which is horse). 
Answer: Literally the border of your hair, the rhyming slang for which is Barnet, from Barnet fair.