The most common and most likely answer to this crossword clue is the 6 letter word BICEPS
I've seen this crossword clue in Financial Times
B
I
C
E
P
S
More clues leading to the result BICEPS
- Muscle energy and power needed in last half of aerobics
- Muscle oil company's used to guard diamonds
- Muscle initially bulging in chaps exercising pretty solidly
- Bishop not half cornering church power and muscle
- Writer tied to bedpost from time to time? It shows flexibility!
- Twice contracting fungal growth in the arm
- They pull up Black Isle mushrooms
- Weightlifters use them to purchase fungi by phone
- Arms workers?
- Manifestation of armed power
Buddy explains!

The straight part
We know that "It shows flexibility" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it shows flexibility = biceps.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 BICEPS mean?
BICEPS noun- Any muscle having two heads.
- Specifically, the biceps brachii, the flexor of the elbow.
- The upper arm, especially the collective muscles of the upper arm.
- A point in a metrical pattern that can be filled either with one long syllable (a longum) or two short syllables (two brevia)