Only personal impressions but admissible as evidence (12)
I believe the answer is:
fingerprints
'only personal impressions' is the definition.
The answer and definition can be both human attributes as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
'admissible as evidence' is the wordplay.
I cannot really understand how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is present in the answer.
This explanation may well be incorrect...
'but' acts as a link.
Can you help me to learn more?
(Another definition for fingerprints that I've seen is " Marks of individuality".)