Lifetime of bridge? (4)
I believe the answer is:
span
This clue is a double definition.
'lifetime' is the first definition.
(I've seen this in another clue)
'bridge?' is the second definition.
(synonyms)
'of' acts as a link.
(Other definitions for span that I've seen before include "Extent of 20" , "Distance or interval between two points" , "Duration, length" , "Distance between two points in time or place" , "The total spread" .)