The most common and most likely answer to this crossword clue is the 8 letter word ORNATELY
I've seen this crossword clue in The Guardian
O
R
N
A
T
E
L
Y
More clues leading to the result ORNATELY
- Knight dined in airport in sumptuous manner
- In the rococo style, worn at Élysée gathering . . . .
- No service in cathedral city? Plainly not!
- Len, a Tory, goes out in florid style
- Not really elaborate, lacking one line in elaborate style
- In highly decorated style or neatly arranged
- Later on, dramatically start to yell in a showy manner
Buddy explains!
The straight part
We know that "Plainly not" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plainly not = ornately.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.