Capital energy for every source of heat (7,6)
I believe the answer is:
cayenne pepper
'source of heat' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'capital energy for every' is the wordplay.
'capital' becomes 'cayenne' (I can't justify this - if you can you should believe this answer much more).
'energy' becomes 'pep' (personal energy, vim).
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'cayenne'+'pep'+'per'='CAYENNE-PEPPER'
Can you help me to learn more?
(Other definitions for cayenne pepper that I've seen before include "Pungent spice" , "Hot condiment" , "Powder from capsicums" .)