It's Wednesday, so duck a dunderhead in the river! (5)
I believe the answer is:
today
'it's wednesday' is the definition.
I can't tell whether this definition defines the answer.
'duck a dunderhead in the river' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'a dunderhead' becomes 'd' (head letter of 'dunder').
'in the' indicates putting letters inside.
'river' becomes 'tay' (Scottish river).
'o'+'d'='od'
'od' placed inside 'tay' is 'TODAY'.
'so' acts as a link.
Can you help me to learn more?
(Other definitions for today that I've seen before include "Yesterday's tomorrow" , "Current period of 24 hours" , "Nowadays" , "The present time or age" , "At the present time, in general" .)