Very old bridge, one linking capitals of Austria and Czechia (7)
I believe the answer is:
archaic
'very old' is the definition.
(I've seen this in another clue)
'bridge one linking capitals of austria and czechia' is the wordplay.
'bridge' becomes 'arch' (I've seen this before).
'one linking' says to put letters next to each other.
'capitals of' indicates taking the first letters.
'and' is a charade indicator (letters next to each other).
'czechia' becomes 'ic' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'austria' is 'a'.
'arch'+'a'+'ic'='ARCHAIC'
Can you help me to learn more?
(Other definitions for archaic that I've seen before include "Belonging to former times, perhaps word usage" , "Ancient or out of date" , "Belonging to former or ancient times" , "Antiquated" , "A long way out of date" .)