The most common and most likely answer to this crossword clue is the 11 letter word UNCONFESSED
I've seen this crossword clue in The Times
U
N
C
O
N
F
E
S
S
E
D
Buddy explains!

The straight part
We know that "not declared" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not declared = unconfessed.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 UNCONFESSED mean?
UNCONFESSED adjective- Not acknowledged
- Not confessed (to a priest)