The most common and most likely answer to this crossword clue is the 5 letter word RALLY
I've seen this crossword clue in The Guardian
R
A
L
L
Y
More clues leading to the result RALLY
- Right person to give support in meeting
- Genuinely devoid of energy in comeback
- Start to recover from exchange of shots
- Take supporter’s banter
- Very last of troupe to quit demonstration
- Run to join forces and gain strength
- Improve court practice?
- Improve some court action?
- Turn the corner in driving contest
- Sir Walter casually said: “Convention”
Buddy explains!

The straight part
We know that "demonstration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demonstration = rally.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 RALLY mean?
RALLY noun- A public gathering or mass meeting that is not mainly a protest and is organized to inspire enthusiasm for a cause.
- A protest or demonstration for or against something, but often with speeches and often without marching, especially in North America.
- A sequence of strokes between serving and scoring a point.
- An event in which competitors drive through a series of timed special stages at intervals. The winner is the driver who completes all stages with the shortest cumulative time.
- A recovery after a decline in prices (said of the market, stocks, etc.)
- To collect, and reduce to order, as troops dispersed or thrown into confusion; to gather again; to reunite.
- To come into orderly arrangement; to renew order, or united effort, as troops scattered or put to flight; to assemble; to unite.
- To collect one's vital powers or forces; to regain health or consciousness; to recuperate.
- To recover strength after a decline in prices; -- said of the market, stocks, etc.
- Good-humoured raillery.
- To tease; to chaff good-humouredly.