The most common and most likely answer to this crossword clue is the 14 letter word MEPHISTOPHELES
I've seen this crossword clue in The Sunday Telegraph
M
E
P
H
I
S
T
O
P
H
E
L
E
S
More clues leading to the result MEPHISTOPHELES
- Devil of a politician leading awful sheepish lot
- Home help with pets is arranged for evildoer
- One in HMP stole sheep originally, the devil!
- The devil, he helps me out, keeping one close
- The hopeless MP I upset is diabolical character
- Representative in Strasbourg, the man’s best man, mostly minor person with soul-destroying job?
Buddy explains!
The straight part
We know that "diabolical character" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diabolical character = mephistopheles.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.