Time to impersonate a duck? Nonsense! (7)
I believe the answer is:
twaddle
'nonsense' is the definition.
(twaddle is a kind of nonsense)
'time to impersonate a duck?' is the wordplay.
'time' becomes 't' (abbreviation).
'to impersonate a duck?' becomes 'waddle' (ducks waddle when they walk).
't'+'waddle'='TWADDLE'
(Other definitions for twaddle that I've seen before include "Moonshine" , "Balderdash" , "Senseless talk" , "Foolish words" , "tripe" .)