The main obstacle between Britain and Europe (7)
I believe the answer is:
channel
'the main' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for channel that I've seen before include "Serve as a medium for (a spirit)" , "Length of water; radio station" , "Gutter; medium" , "Ditch" , "Furrow" .)