The most common and most likely answer to this crossword clue is the 10 letter word COMPETENCE
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More clues leading to the result COMPETENCE
Buddy explains!

The straight part
We know that "fitness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fitness = competence.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 COMPETENCE mean?
COMPETENCE noun- The quality or state of being competent, i.e. able or suitable for a general role.
- The quality or state of being able or suitable for a particular task; the quality or state of being competent for a particular task.
- The system of linguistic knowledge possessed by native speakers of a language, as opposed to its actual use in concrete situations (performance), cf. :w:linguistic competence.
- A sustainable income.
- The legal authority to deal with a matter.
- The degree to which a rock is resistant to deformation or flow.