Cut of meat: stuff the thing they say (6)
I believe the answer is:
fillet
'cut of meat' is the definition.
(fillet is a kind of cut of meat)
'stuff the thing they say' is the wordplay.
I cannot quite understand how this works, but
'stuff' could be 'fill' (synonyms) and 'fill' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fillet that I've seen before include "Remove bones of (fish)" , "Cut of steak; ribbon strip" , "Juicy piece of boneless meat" , "Remove bone from (meat or fish)" , "Boneless slice, of meat say" .)