The most common and most likely answer to this crossword clue is the 6 plus 3 letter word BATTLE CRY
I've seen this crossword clue in The Guardian
B
A
T
T
L
E
C
R
Y
More clues leading to the result BATTLE CRY
Buddy explains!
The straight part
We know that "Keen for action" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keen for action = battle cry.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.