The most common and most likely answer to this crossword clue is the 7 letter word CLEARLY
I've seen this crossword clue in Financial Times
C
L
E
A
R
L
Y
More clues leading to the result CLEARLY
- 150 — very old, without doubt
- Prematurely supporting gutless colonel, of course
- Many in good time plainly seen
- Conservative soon eclipsing left, without doubt
- As is evident, its heart is quite a different organ
- Without doubt, leader of competition left in good time
- Variable recall: variable without doubt
- Head of college left prematurely, evidently
- Without doubt, Charlie – King – gets special day ultimately
Buddy explains!

The straight part
We know that "plainly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case plainly = clearly.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 CLEARLY mean?
CLEARLY adverb- (manner) In a clear manner.
- (modal) Without a doubt; obviously.
- (degree) To a degree clearly discernible.