Basic source of heat - a pound (9)
I believe the answer is:
elemental
'basic' is the definition.
(synonyms)
'source of heat a pound' is the wordplay.
'source of heat' becomes 'element' (part of an electronic device which provides heat).
'pound' becomes 'L' (the pound sign is based on an L).
'element'+'a'+'l'='ELEMENTAL'
(Other definitions for elemental that I've seen before include "Let me lean like forces of nature" , "of fire, say" , "Pertaining to weather or to first principles" , "Of basic importance" , "being like Mercury?" .)