A, B, C etc, as periodically arranged? He and I are with them (8)
I believe the answer is:
elements
'a b c etc' is the definition.
'periodically arranged? he and i are with them' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'lem' and 'lem' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'as' acts as a link.
(Other definitions for elements that I've seen before include "First principles of the weather" , "Nitrogen and oxygen, e.g." , "Constituent ingredients" , "Components, factors" , "Carbon, iron and 8 Across, say" .)