The most common and most likely answer to this crossword clue is the 9 letter word PREOCCUPY
I've seen this crossword clue in The Times
P
R
E
O
C
C
U
P
Y
Buddy explains!
The straight part
We know that "Concern" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concern = preoccupy.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 PREOCCUPY mean?
PREOCCUPY verb- To distract; to occupy or draw attention elsewhere.
- To occupy or take possession of beforehand.