Main road, one passing through channel (8)
I believe the answer is:
cardinal
'main' is the definition.
(thesaurus)
'road one passing through channel' is the wordplay.
'road' becomes 'rd' (abbreviation).
'one' becomes 'i' (Roman numeral).
'passing through' indicates putting letters inside.
'channel' becomes 'canal' (canal is a kind of channel).
'rd'+'i'='rdi'
'rdi' inserted into 'canal' is 'CARDINAL'.
(Other definitions for cardinal that I've seen before include "Type of number or prelate" , "Counsellor to the Pope" , "Main point of compass - most important" , "Senior Roman Catholic" , "Primary - colour" .)