The most common and most likely answer to this crossword clue is the 14 letter word IDENTIFICATION
I
D
E
N
T
I
F
I
C
A
T
I
O
N
More clues leading to the result IDENTIFICATION
Buddy explains!

The straight part
We know that "diagnosis" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case diagnosis = identification.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.
Crossword Answer definitions
IDENTIFICATION noun- The act of identifying, or proving to be the same.
- The state of being identified.
- A particular instance of identifying something.
- A document or documents serving as evidence of a person's identity.
- A feeling of support, sympathy, understanding or belonging towards somebody or something.