The most common and most likely answer to this crossword clue is the 12 letter word SWASHBUCKLER
I've seen this crossword clue in The Times
S
W
A
S
H
B
U
C
K
L
E
R
Buddy explains!

The straight part
We know that "Swaggering adventurer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swaggering adventurer = swashbuckler.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 SWASHBUCKLER mean?
SWASHBUCKLER noun- A swordsman or fencer who engages in showy or extravagant sword play.
- A daring adventurer.
- A kind of period adventure story with flashy action and lighthearted tone.