Mix 3 ingredients for sandwich filling (4)
I believe the answer is:
tuna
This might be a double definition.
'mix 3 ingredients' is the first definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'sandwich filling' is the second definition.
(tuna is a common sandwich filling)
'for' is the link.
(Other definitions for tuna that I've seen before include "Marine food for your strange aunt" , "Tunny" , "Fish, often tinned" , "Fish; edible cactus" , "Warm-water food fish" .)