The most common and most likely answer to this crossword clue is the 8 letter word EMPHATIC
I've seen this crossword clue in The Guardian
E
M
P
H
A
T
I
C
More clues leading to the result EMPHATIC
- Forceful English politician in charge should admit pork pie maybe
- What about politician with a muscular movement being decisive?
- Impressive space heater originally commandeered by girl in charge
- By stripping, tempt that jerk to become forceful
- Decisive impact he could produce
- The camp I represented is clearly stressed
- Forceful energy and speed at one hundred
- Time chap’s got upset showing stress?
- Confused chap: ‘I’m the missing husband, that’s definite‘
- Impressive meat and chip stew
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 = emphatic.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 EMPHATIC mean?
EMPHATIC noun- An emphatic consonant.
- A word or phrase adding emphasis, such as "a lot" or "really".
- Characterized by emphasis; forceful.
- Stated with conviction.
- (grammar) Belonging to a set of English tense forms comprising the auxiliary verb do + an infinitive without to.
- Belonging to a series of obstruent consonants in several Semitic languages that are distinguished from both voiced and voiceless consonants by a certain phonetic feature or features.