They say it's a direct waterway (6)
I believe the answer is:
strait
'waterway' is the definition.
(I know that strait is a type of waterway)
'they say it's a direct' is the wordplay.
'they say' shows a homophone (sound like).
'it's a direct' becomes 'straight' (I've seen this before).
'straight' sounds like 'STRAIT'.
(Other definitions for strait that I've seen before include "Narrow stretch of sea" , "watery scene?" , "of Gibraltar, for example" , "Connecting passage of water" , "Narrow stretch of water for artist" .)