Rancid meat, a smaller piece thereof (6)
I believe the answer is:
offcut
'a smaller piece thereof' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'rancid meat' is the wordplay.
'rancid' becomes 'off' (I've seen this before).
'meat' becomes 'cut' (cut is a kind of meat).
'off'+'cut'='OFFCUT'
(Other definitions for offcut that I've seen before include "Remnant of material" , "Small bit of some material" , "bit of stick?" , "Piece of material left over" , "Waste piece" .)