Support for line I found in European capital mostly — about time (7)
I believe the answer is:
viaduct
'support for line' is the definition.
(structure supporting a railway)
'i found in european capital mostly about time' is the wordplay.
'found in' is an insertion indicator.
'european capital' becomes 'Vaduz' (capital of Liechtenstein).
'mostly' means to remove the last letter.
'about time' becomes 'ct' (I can't justify this - if you can you should believe this answer much more).
'vaduz' with its final letter taken off is 'vadu'.
'i' going inside 'vadu' is 'viadu'.
'viadu'+'ct'='VIADUCT'
(Other definitions for viaduct that I've seen before include "Road/rail bridge" , "Long bridge over land" , "Type of bridge" , "Bridge across a valley" , "Rail bridge over valley" .)