The most common and most likely answer to this crossword clue is the 9 letter word SPRIGHTLY
I've seen this crossword clue in The Guardian
S
P
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L
Y
More clues leading to the result SPRIGHTLY
- Lively cunning grabs power completely
- In retirement, I would add, fairly active
- Atmosphere in the centre fairly lively
- Nimble American operative taking both sides in
- Energetic agent taking in both sides
- Starting price fairly active
- Active agent accepts true-blue line
- Full of vitality and roguish seizing power? True
- Spirited soldier at the helm properly guarding president
Buddy explains!

The straight part
We know that "Nimble" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nimble = sprightly.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 SPRIGHTLY mean?
SPRIGHTLY adjective- Animated, gay or vivacious; lively, spirited.
- Of a person: full of life and vigour, especially with a light and springy step.
- Of or relating to a sprite; ghostly, spectral.
- In a lively and vigorous way; sprightlily.