Helium, say, on a lake, is basic (9)
I believe the answer is:
elemental
'basic' is the definition.
(synonyms)
'helium say on a lake' is the wordplay.
'helium say' becomes 'element' (helium is a kind of element).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'lake' becomes 'L' (geographical abbreviation).
'element'+'a'+'l'='ELEMENTAL'
'is' is the link.
(Other definitions for elemental that I've seen before include "Of nature's powers" , "Rising trend" , "Basic, primitive" , "Of basic importance" , "radical" .)