The most common and most likely answer to this crossword clue is the 6 letter word NESTED
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More clues leading to the result NESTED
Buddy explains!

The straight part
We know that "fitted together" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fitted together = nested.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 NESTED mean?
NESTED verb- (of animals) To build or settle into a nest.
- To settle into a home.
- To successively neatly fit inside another.
- To place in, or as if in, a nest.
- To place one thing neatly inside another, and both inside yet another (and so on).
- To hunt for birds' nests or their contents (usually "go nesting").
- Embedded.
- Successively fitted one inside another.
- (Of a lexical item) contained within a dictionary entry as a subordinate term of the main headword.