De Niro exploded with "cut, cut!" (9)
I believe the answer is:
reduction
'cut' is the definition.
(I've seen this before)
'de niro exploded with cut' is the wordplay.
'exploded with' is an anagram indicator.
'de'+'niro'='deniro'
'deniro' mixed up with 'cut' gives 'REDUCTION'.
(Other definitions for reduction that I've seen before include "thickened sauce?" , "Drop in price" , "Lessening" , "cutback!" , "Introduce (anag) - decrease" .)