The most common and most likely answer to this crossword clue is the 8 letter word NIMBLEST
I've seen this crossword clue in The Daily Telegraph
N
I
M
B
L
E
S
T
More clues leading to the result NIMBLEST
Buddy explains!

The straight part
We know that "most active" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most active = nimblest.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 NIMBLEST mean?
NIMBLEST adjective- Adept at taking or grasping
- Quick and light in movement or action.
- Quick-witted and alert.