Crossing by way of tube (7)
I believe the answer is:
viaduct
'crossing' is the definition.
(thesaurus)
'by way of tube' is the wordplay.
'by way of' becomes 'via' (I've seen this before).
'tube' becomes 'duct' (I've seen this before).
'via'+'duct'='VIADUCT'
(Other definitions for viaduct that I've seen before include "Long bridge over land" , "Bridge carrying a road over a river or valley" , "Long arched structure" , "Bridge-like structure" , "Bridge to carry railway over a valley" .)