The most common and most likely answer to this crossword clue is the 3 plus 2 letter word ACT UP
I've seen this crossword clue in The Times
A
C
T
U
P
More clues leading to the result ACT UP
- Behave badly at university after play
- Roughly flipped sheep muck about
- Be a nuisance, wanting tea say not in a mug?
- Behave badly, as a court’s risen
- Performance in contention is to go wrong
- Malfunction leads to Bill getting deposit back
- Bill’s sheep giving trouble
- Behave badly in pretence at college
- Bill gets sheep to behave badly
- Misbehave in a court before judge?
Buddy explains!

The straight part
We know that "Be a nuisance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be a nuisance = act up.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.