The most common and most likely answer to this crossword clue is the 6 letter word WEIRDO
I've seen this crossword clue in Financial Times
W
E
I
R
D
O
More clues leading to the result WEIRDO
- Queer fish in dam do
- Oddball’s party by dam
- Odd fish in dam as before
- I rowed out to find queer fish
- Fish trap on river gathering the odd fish?
- Eccentric person needing low dam to perform
- Eccentric widower devastated after losing wife
- Word I supply to encapsulate eccentric principally?
- Freak party under waterfall
- Loose-wired ring card
Buddy explains!

The straight part
We know that "Word I supply to encapsulate eccentric principally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case word i supply to encapsulate eccentric principally = weirdo.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 WEIRDO mean?
WEIRDO noun- A strange, odd, eccentric person.
- An insane, possibly dangerous person.