"Fiddle-faddle!" (5)
(Other definitions for pshaw that I've seen before include "Expression of contempt" , "scornful reaction" , "Contemptuous riposte" , "Contemptuous cry" , "Voice impatience" .)
'fiddle-faddle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nonsense that I've seen before include "Bull" , "folly" , "rhubarb" , "Gibberish" , "Piffle and poppycock" .)