Fatty tissue? Some tissue treated (4)
I believe the answer is:
suet
'fatty tissue?' is the definition.
(I've seen this before)
'some tissue treated' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SUET' is hidden within 'tissue treated'.
(Other definitions for suet that I've seen before include "Hard animal fat used in cooking" , "Hard fat in cattle and sheep" , "Fatty tissue" , "Solid fat used in cooking" , "Edible animal fat" .)