Not, you say, a winding passage (6)
I believe the answer is:
strait
'passage' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for strait that I've seen before include "Narrow stretch of water as at Gibraltar" , "Narrow strip of water between two larger ones" , "Narrow stretch of water for artist" , "Narrow waterway" , "Narrow channel of the sea joining two larger bodies of water" .)