The most common and most likely answer to this crossword clue is the 12 letter word ENTHUSIASTIC
I've seen this crossword clue in The Daily Telegraph
E
N
T
H
U
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I
A
S
T
I
C
More clues leading to the result ENTHUSIASTIC
- Accordingly dressed in pants in case it's warm
- Sauciest hint when frolicking making one keen
- I fiddled with scanties, thus endlessly eager
- Into cuisine that’s foreign?
- Vehement cheat’s in US it turned out
- Chastise unit at sea that’s keen
- Sauciest hint changed, becoming zealous
- Eager to see thin, sauciest pants
- Cuisine that’s exciting and lively
- Eager moving round cities’ haunts
Buddy explains!

The straight part
We know that "Vehement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vehement = enthusiastic.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 ENTHUSIASTIC mean?
ENTHUSIASTIC adjective- With zealous fervor; excited, motivated.