Bridge completed? I don't know (8)
I believe the answer is:
overpass
'bridge' is the definition.
(overpass is a kind of bridge)
'completed? i don't know' is the wordplay.
'completed?' becomes 'over' (synonyms).
'i don't know' becomes 'pass' ('pass' is said to skip a question).
'over'+'pass'='OVERPASS'
(Other definitions for overpass that I've seen before include "High-level road" , "bridge" , "high road" , "Flyover" .)