The most common and most likely answer to this crossword clue is the 7 letter word CONFIRM
I've seen this crossword clue in The Times
C
O
N
F
I
R
M
More clues leading to the result CONFIRM
- Artifice hard to prove
- Verify clubs working with company
- Establish first of cricketers in form after knock
- Fix fiddle with timber beginning to mould
- Administer religious rite to defraud business
- Establish the accuracy of indefinite figure invested in two companies?
- Establish company in the wake of deception
- Authorise fraud on company
- Establish cheat over company
- Strengthen company with new business
Buddy explains!

The straight part
We know that "Declare, formally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case declare, formally = confirm.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 CONFIRM mean?
CONFIRM verb- To strengthen; to make firm or resolute.
- To administer the sacrament of confirmation on (someone).
- To assure the accuracy of previous statements.