The most common and most likely answer to this crossword clue is the 5 letter word ARSES
A
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S
More clues leading to the result ARSES
Buddy explains!

The straight part
We know that "they’re upbeat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re upbeat = arses.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 ARSES mean?
ARSES noun- The buttocks or more specifically, the anus.
- A stupid, mean or despicable person.
- To be silly, act stupid or mess around.
- The stronger part of a musical measure: the part containing the beat.
- The stronger part of a metrical foot: the part containing the long (heavy) syllable in quantitative meter, or the stressed syllable in a qualitative meter.
- The elevation of the hand, or that part of the bar at which it is raised, in beating time; the weak or unaccented part of the bar, opposed to the thesis.
- The elevation of the voice to a higher pitch in speaking.