In the first instance, Biden's age is a striking characteristic (8)
I believe the answer is:
boldness
'a striking characteristic' is the definition.
'in the first instance biden's age' is the wordplay.
'in the first instance' indicates taking the first letters.
'age' becomes 'oldness' (oldness is a kind of age).
The first letter of 'biden' is 'b'.
'b'+'oldness'='BOLDNESS'
'is' acts as a link.
(Other definitions for boldness that I've seen before include "brass" , "Audacity" , "Confidence, daring" , "Courage, daring" .)