### Clue: Believe person is without a relationship (5)
### Answer: OPINE 
 ### Explanation: Definition: Believe
  ONE(an individual,person) **containing**(without) PI(the relationship,ratio = circumference of a circle divided by its diameter, a decimal number that people actually vie to memorise.  Guinness record holder has got it to 67890 digits, and there's an unconfirmed feat of 100,000 digits.  Talk about single-mindedness.  Me, I'm happy with 3.142 or even 22/7)