The most common and most likely answer to this crossword clue is the 6 letter word SHAKOS
S
H
A
K
O
S
Buddy explains!
The straight part
We know that "Plumed caps" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plumed caps = shakos.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 SHAKOS mean?
SHAKOS noun- A stiff, cylindrical military dress hat with a metal plate in front, a short visor, and a plume.
- A bearskin or busby.
- The squilla or mantis shrimp.