Flatten flatfish, so it's said (4)
I believe the answer is:
raze
'flatten' is the definition.
(thesaurus)
'flatfish so it's said' is the wordplay.
'flatfish' becomes 'rays' (flatfish are a type of ray).
'so it's said' indicates a 'sounds like' (homophone) clue.
'rays' is a homophone of 'RAZE'.
(Other definitions for raze that I've seen before include "Burn to the ground" , "Destroy completely, tear down to the ground" , "Demolish completely" , "level" , "Demolish (a building)" .)