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

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