The most common and most likely answer to this crossword clue is the 13 letter word PREOCCUPATION
I've seen this crossword clue in The Daily Telegraph
P
R
E
O
C
C
U
P
A
T
I
O
N
Buddy explains!

The straight part
We know that "Concentration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concentration = preoccupation.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 PREOCCUPATION mean?
PREOCCUPATION noun- The state of being preoccupied or an idea that preoccupies the mind; enthrallment.
- The act of occupying something before someone else.