Duck backing away from big river (3)
I believe the answer is:
nil
'duck' is the definition.
(both can mean zero)
'backing away from big river' is the wordplay.
'backing away from' means to remove the last letter (the back letter is removed).
'big river' becomes 'Nile' (River Nile).
'nile' with its last letter taken away is 'NIL'.
(Other definitions for nil that I've seen before include "Score of nought" , "You get nothing" , "Non-existent" , "Nothing, zero" , "Zero, no score" .)