Old tidal wave eliminating river reed (4)
I believe the answer is:
oboe
'reed' is the definition.
(oboe is a kind of reed)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for oboe that I've seen before include "Player" , "Woodwind instrument of treble pitch" , "Double-reed woodwind" , "Double-reed wind instrument" , "Instrument used in 1d" .)