The most common and most likely answer to this crossword clue is the 12 letter word RARINGÂ TOÂ GO
I've seen this crossword clue in The Guardian
R
A
R
I
N
G
Â
T
O
Â
G
O
Buddy explains!

The straight part
We know that "Gung-ho" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gung-ho = raring to go.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.