Basic source of heat by a lake (9)
I believe the answer is:
elemental
'basic' is the definition.
(synonyms)
'source of heat by a lake' is the wordplay.
'source of heat' becomes 'element' (part of an electronic device which provides heat).
'by' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'element'+'a'+'l'='ELEMENTAL'
(Other definitions for elemental that I've seen before include "Of nature's powers" , "Related to the powers of nature" , "being like Mercury?" , "Let me lean like forces of nature" , "Basic, primitive" .)