The most common and most likely answer to this crossword clue is the 11 letter word DEMONSTRATE
I've seen this crossword clue in The Guardian
D
E
M
O
N
S
T
R
A
T
E
More clues leading to the result DEMONSTRATE
- Prove moves not mastered
- Devil treats suspect to show
- Show how Devil and Saint rank in esteem
- Show how devil and saint rank in estimation
- Review after seeing ‘Devil and Saint’ show
- Show leader of race in great condition
- Fiendish types are reportedly in gallery for show
- … villain’s given time by judge for show
- Show modern state to be in need of reform
- Show fanatical holy man before judge
Buddy explains!

The straight part
We know that "show" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case show = demonstrate.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 DEMONSTRATE mean?
DEMONSTRATE verb- To show how to use (something).
- To show the steps taken to create a logical argument or equation.
- To participate in or organize a demonstration.
- To show, display, or present; to prove or make evident