The most common and most likely answer to this crossword clue is the 6 letter word ORDAIN
I've seen this crossword clue in The Guardian
O
R
D
A
I
N
More clues leading to the result ORDAIN
- Demand or be condescending in speech
- Stipulate quantity to be eaten daily in following paleo, ultimately
- Alternatively, row about a decree
- Other ranks row about a decree
- Make a priest or old monk take one in
- Invest in road building
- Will ring road join road north?
- Admit to being a priest
- Declare Rodin a lunatic
- Establish insomnia
Buddy explains!

The straight part
We know that "Declare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declare = ordain.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 ORDAIN mean?
ORDAIN verb- To prearrange unalterably.
- To decree.
- To admit into the ministry of a religion, for example as a priest, bishop, minister or Buddhist monk, or to authorize as a rabbi.
- To predestine.