The most common and most likely answer to this crossword clue is the 5 letter word ANODE
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The straight part
We know that "essential part of circuit?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case essential part of circuit? = anode.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ANODE mean?
ANODE noun- An electrode, of a cell or other electrically polarized device, through which a positive current of electricity flows inwards (and thus, electrons flow outwards). It can have either a negative or a positive voltage.
- (by extension) The electrode at which chemical oxidation of anions takes place, usually resulting in the erosion of metal from the electrode.
- The electrode which collects electrons emitted by the cathode in a vacuum tube or gas-filled tube.
- That electrode of a semiconductor device which is connected to the p-type material of a p-n junction.