Stream of purest water (3)
I believe the answer is:
ure
'stream' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'of purest water' is the wordplay.
'of' indicates a hidden word.
'URE' is hidden in the letters of 'purest water'.
Can you help me to learn more?
(Other definitions for ure that I've seen before include "Ripon's river" , "Midge --, singer" , "River of N. Yorkshire" , "North Yorkshire river" , "Wensleydale river" .)