Overpass by the Tube (7)
I believe the answer is:
viaduct
'overpass' is the definition.
(thesaurus)
'by the tube' is the wordplay.
'by' becomes 'via' (travelling by or via a place).
'the tube' becomes 'duct' (type of tube or canal).
'via'+'duct'='VIADUCT'
(Other definitions for viaduct that I've seen before include "Bridge across a valley" , "Road/rail bridge" , "Long bridge over land" , "Bridge carrying a road over a river or valley" , "Bridge-like structure" .)