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

The straight part
We know that "Diagnostic test" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diagnostic test = biopsy.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 BIOPSY mean?
BIOPSY noun- The removal and examination of a sample of tissue, cells, or bodily fluid from a living body for diagnostic purposes.
- To take a sample (a biopsy) for pathological examination.