Purely encircling waterway (3)
I believe the answer is:
ure
'waterway' is the definition.
(The River Ure in North Yorkshire)
'purely encircling' is the wordplay.
'encircling' indicates the answer is hidden within the clue.
'URE' is hidden in the letters of 'purely'.
(Other definitions for ure that I've seen before include "North Yorkshire river" , "Wensleydale river" , "River at Ripon" , "River of N. Yorkshire" , "Ripon's river" .)