A first principle in current heaters (7)

I believe the answer is:

element

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'a first principle' is the definition.
'element' can be an answer for 'principle' (thesaurus). I am not certain of the 'a first' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for element that I've seen before include "For example, gold" , "that's used in heating" , "Fundamental part of whole" , "Essential factor" , "Constituent -- habitat" .)

I've seen this clue in The Irish News.

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