Source of tallow (4)
I believe the answer is:
suet
'source of tallow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for suet that I've seen before include "Hard white fat on sheep's kidneys" , "Fatty stuff used in cooking" , "Edible fat used in cooking" , "Thin pancake" , "Edible animal fat" .)