The most common and most likely answer to this crossword clue is the 7 letter word MOBSTER
I've seen this crossword clue in The Daily Telegraph
M
O
B
S
T
E
R
More clues leading to the result MOBSTER
- Principal substituting old boy for a family man
- Mafioso beast taking out knight for bishop
- Teacher’s article replaced by former pupil’s hood
- Tough seafood, starter changed
- Male crustacean with head hidden in hood
- Medium crustacean, not large? Criminal
- Male crustacean missing head and hood
- Second best criminal runs for hood
- Original film with old boy knocking out a US criminal
- Dragon needs start of battle for knight, one in 25
Buddy explains!

The straight part
We know that "Mafioso" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mafioso = mobster.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 MOBSTER mean?
MOBSTER noun- A member of a mob or Mafia