The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SEALED ORDERS
I've seen this crossword clue in The Times
S
E
A
L
E
D
O
R
D
E
R
S
More clues leading to the result SEALED ORDERS
Buddy explains!
The straight part
We know that "Secret plans" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secret plans = sealed orders.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.