The most common and most likely answer to this crossword clue is the 8 letter word SOLENOID
I've seen this crossword clue in The Times
S
O
L
E
N
O
I
D
More clues leading to the result SOLENOID
Buddy explains!

The straight part
We know that "Coil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coil = solenoid.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 SOLENOID mean?
SOLENOID noun- A coil of wire that acts as a magnet when an electric current flows through it.
- A mechanical switch consisting of such a coil containing a metal core, the movement of which is controlled by the current.