The most common and most likely answer to this crossword clue is the 6 letter word IMPROV
I've seen this crossword clue in Independent
I
M
P
R
O
V
More clues leading to the result IMPROV
- Finish off better stand-up routine
- Rally, in short, not taking the rehearsed route
- Unprepared stuff the writer’s both for and against
- I am in favour of verse that’s ad-lib
- Spontaneous performance mostly better
- Finish off better stage technique
- Taking drug, get better for unrehearsed performance
- The speaker’s for and against ad-libbing
- Little devil, third in show, brought in Bible for ad libbing
- Comic performance better finishing early
Buddy explains!

The straight part
We know that "unrehearsed performance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unrehearsed performance = improv.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 IMPROV mean?
IMPROV noun- Improvisation.
- A form of live entertainment characterized by improvisation and interaction with the audience.
- To perform improv.