The most common and most likely answer to this crossword clue is the 6 letter word BOWELS
I've seen this crossword clue in The Guardian
B
O
W
E
L
S
Buddy explains!

The straight part
We know that "Interior" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case interior = bowels.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 BOWELS mean?
BOWELS noun- A part or division of the intestines, usually the large intestine.
- (in the plural) The entrails or intestines; the internal organs of the stomach.
- (in the plural) The (deep) interior of something.
- (in the plural) The seat of pity or the gentler emotions; pity or mercy.
- (in plural) offspring
- The deepest or innermost part.
- The concept or quality that defines something at its very core.
- The intestines.
- Compassion, sympathy.
- The body as the source of offspring.