Potato dish consumed in quiet: it’s a great success! (5,3)
I believe the answer is:
smash hit
'a great success' is the definition.
(a smash hit is a commercial success)
'potato dish consumed in quiet it's' is the wordplay.
'potato dish' becomes 'mash' (mashed potato).
'consumed in' is an insertion indicator.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'mash' inserted within 'sh' is 'smashh'.
'smashh'+'it'='SMASH-HIT'
(Other definitions for smash hit that I've seen before include "Huge success" , "Really successful song" , "blockbuster" , "Major success" , "number one" .)