The most common and most likely answer to this crossword clue is the 9 plus 4 letter word INDUCTION COIL
I've seen this crossword clue in The Times
I
N
D
U
C
T
I
O
N
C
O
I
L
Buddy explains!
The straight part
We know that "Part of circuit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of circuit = induction coil.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.