Chain mail regularly seen in waterway (5)
I believe the answer is:
canal
'waterway' is the definition.
(I know that canal is a type of waterway)
'chain mail regularly seen' is the wordplay.
'regularly seen' means one should take alternating letters.
'chain'+'mail'='chainmail'
The alternate letters of 'chainmail' are 'CANAL'.
'in' is the link.
(Other definitions for canal that I've seen before include "Passage" , "Artificial waterway for barges" , "Long, narrow strip of water" , "Man-made waterway" , "Grand Union perhaps" .)