### Clue: A, B, C etc, as periodically arranged? He and I are with them (8)
### Answer: ELEMENTS 
 ### Explanation: Definition: A, B, 
I think this is a double definition:
 A, B, C etc = 'the ABCs' = 'the basics' = ELEMENTS
 He (Helium) and I (Iodine) are ELEMENTS arranged in the periodic table