The most common and most likely answer to this crossword clue is the 13 letter word UNINTENTIONAL
I've seen this crossword clue in The Daily Telegraph
U
N
I
N
T
E
N
T
I
O
N
A
L
Buddy explains!
The straight part
We know that "not part of the plan" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not part of the plan = unintentional.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.
Crossword Answer definitions
UNINTENTIONAL adjective- Not intended or deliberate; inadvertent; unwitting