The most common and most likely answer to this crossword clue is the 6 letter word MUSCLE
I've seen this crossword clue in The Guardian
M
U
S
C
L
E
More clues leading to the result MUSCLE
- Perhaps calf is a beast of burden carrying sack regularly
- Power of special constable to intercept drug smuggler
- Might head of convent left inside meditate?
- Might millions crack clues?
- Clue: S&M involved power
- Patron of the arts, catching cold, left beef
- Heart, for example, can be heavy
- Might heads of security come to be taken in by drug smuggler?
- Sham US cleric seizing power
- Hippopotamus clearly shows strength
Buddy explains!

The straight part
We know that "beef" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beef = muscle.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 MUSCLE mean?
MUSCLE noun- A contractile form of tissue which animals use to effect movement.
- An organ composed of muscle tissue.
- (usually in the plural) A well-developed physique, in which the muscles are enlarged from exercise.
- Strength, force.
- Hired strongmen or bodyguards.
- To use force to make progress, especially physical force.