Source of heat for situation in which you're most effective (7)
I believe the answer is:
element
I believe this is a double definition.
'source of heat' is the first definition.
(part of an electronic device which provides heat)
'situation in which you're most effective' is the second definition.
(as in being 'in your element')
'for' acts as a link.
(Other definitions for element that I've seen before include "iron or copper, perhaps" , "Chemical building block" , "Operative part of an electric kettle" , "Component - factor" , "piece" .)