Sounds like previous river (3)
I believe the answer is:
exe
'river' is the definition.
(River Exe)
'sounds like previous' is the wordplay.
I cannot really understand how this works, but
'previous' could be 'ex' (prefix meaning former or onetime) and 'ex' is present in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
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" , "Tiverton's river" , "Topsham's river" , "flower" .)