Plan about river is not sound (5)
I believe the answer is:
draft
'plan' is the definition.
(I know this)
'about river is not sound' is the wordplay.
'about' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'is not sound' becomes 'daft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r' inserted inside 'daft' is 'DRAFT'.
Can you help me to learn more?
(Other definitions for draft that I've seen before include "Sketch; money order" , "Preliminary outline" , "Current of air or preliminary version" , "sketch, perhaps" , "Rough drawing" .)