The most common and most likely answer to this crossword clue is the 4 plus 2 plus 5 letter word BABY ON BOARD
I've seen this crossword clue in The Guardian
B
A
B
Y
O
N
B
O
A
R
D
Buddy explains!
The straight part
We know that "Junior executive position/often advertised in the back window" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case junior executive position/often advertised in the back window = baby on board.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.