It flows perpetually between opposite points (6)
I believe the answer is:
severn
'it flows' is the definition.
The definition suggests a singular noun which matches the answer.
'perpetually between opposite points' is the wordplay.
'perpetually' becomes 'ever' (I've seen this in another clue).
'between' is an insertion indicator.
'opposite' says the letters should be written backwards (I've seen 'opposite of' mean this).
'points' becomes 'ns' (two compass points).
'ns' back-to-front is 'sn'.
'ever' going inside 'sn' is 'SEVERN'.
Can you help me to learn more?
(Other definitions for severn that I've seen before include "Britain's longest river" , "Shrewsbury's river" , "Britian's longest river" , "Welsh and English river" , "A major British river" .)