No longer noted as a river? (3)
I believe the answer is:
exe
'a river?' is the definition.
(River Exe)
'no longer noted' is the wordplay.
I cannot quite see how this works, but
'no longer' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'as' acts as a link.
Can you help me to learn more?
(Other definitions for exe that I've seen before include "River flowing from Exmoor to Exmouth via Exeter" , "River in SW UK" , "West Country river" , "Runner" , "Somerset/Devon river" .)