The most common and most likely answer to this crossword clue is the 6 letter word FIRMLY
I've seen this crossword clue in Financial Times
F
I
R
M
L
Y
Buddy explains!
The straight part
We know that "in a decisive manner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a decisive manner = firmly.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 FIRMLY mean?
FIRMLY adverb- (manner) In a firm or definite or strong manner.
- (manner) Securely.