In this port regularly sail over entrance (8)
I believe the answer is:
plymouth
'in this port' is the definition.
'plymouth' can be an answer for 'port' (port in Devon). I am not certain of the remainder of the definition.
'regularly sail over entrance' is the wordplay.
'regularly sail over' becomes 'ply' (ply can mean to regularly travel a certain rout).
'entrance' becomes 'mouth' (eg the mouth of a cave).
'ply'+'mouth'='PLYMOUTH'
(Other definitions for plymouth that I've seen before include "where sailors found" , "that the Pilgrims left" , "Devon port city" , "Port of which Sir Francis Drake was once mayor" , "Conservative Evangelical Christian movement" .)