The most common and most likely answer to this crossword clue is the 9 letter word FASTENING
I've seen this crossword clue in The Daily Telegraph
F
A
S
T
E
N
I
N
G
Buddy explains!

The straight part
We know that "Concentrating" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concentrating = fastening.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 FASTENING mean?
FASTENING verb- To attach or connect in a secure manner.
- To cause to take close effect; to make to tell; to land.
- A hook or similar restraint used to fasten things together; fastener.