Swimming in her waterway (5)
I believe the answer is:
rhine
'waterway' is the definition.
The answer and definition can be both natural objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'swimming in her' is the wordplay.
'swimming' indicates anagramming the letters.
'in'+'her'='inher'
'inher' is an anagram of 'RHINE'.
Can you help me to learn more?
(Other definitions for rhine that I've seen before include "River rising in Alps, with mouth at Rotterdam" , "River, rising in Switzerland and flowing into the North Sea" , "valley" , "fed by 8 [RHUR]" , "Great German river with Lorelei Rock" .)