The most common and most likely answer to this crossword clue is the 8 plus 4 letter word ELECTRIC FIRE
I've seen this crossword clue in The Guardian
E
L
E
C
T
R
I
C
F
I
R
E
More clues leading to the result ELECTRIC FIRE
Buddy explains!
The straight part
We know that "Plug-in device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plug-in device = electric fire.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.