Helium, say, on a lake, is atmospheric (9)
I believe the answer is:
elemental
'atmospheric' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'helium say on a lake' is the wordplay.
'helium say' becomes 'element' (helium is a kind of element).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'lake' becomes 'L' (geographical abbreviation).
'element'+'a'+'l'='ELEMENTAL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for elemental that I've seen before include "Of nature's powers" , "Essential" , "Of basic importance" , "Rising trend" , "Let me lean like forces of nature" .)