The most common and most likely answer to this crossword clue is the 9 letter word PROFESSOR
I've seen this crossword clue in The Daily Telegraph
P
R
O
F
E
S
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O
R
More clues leading to the result PROFESSOR
- Declare love and, primarily, respect for a teacher
- Principal lecturer's claim disheartened Oscar
- One who claims poor serfs must be radicalised
- Teacher poses for shot with Queen
- One’s given a chair for female – terribly sore, clutching bottom in distress
- Person in chair varying poses for painter, ultimately
- Academic posers for unravelling
- Reporters engaged in besetting of alternative academic
- I have a chair, I declare
- Posers for supply teacher
Buddy explains!

The straight part
We know that "I have a chair/I declare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i have a chair/i declare = professor.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 PROFESSOR mean?
PROFESSOR noun- The most senior rank for an academic at a university or similar institution, informally also known as "full professor." Abbreviated Prof.
- A teacher or faculty member at a college or university regardless of formal rank.
- One who professes something, such as a religious doctrine.
- A pianist in a saloon, brothel, etc.
- The puppeteer who performs a Punch and Judy show; a Punchman.