The most common and most likely answer to this crossword clue is the 6 letter word INMOST
I've seen this crossword clue in The Times
I
N
M
O
S
T
Buddy explains!

The straight part
We know that "as close to the centre as possible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as close to the centre as possible = inmost.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 INMOST mean?
INMOST noun- That which is innermost; the core.
- The very deepest within; farthest from the surface or external part; innermost