Sauce to either side of mozzarella, but not in the middle (6)
I believe the answer is:
tomato
'sauce' is the definition.
(tomato sauce is a kind of sauce)
'to either side of mozzarella but not in the middle' is the wordplay.
'either side of' is an insertion indicator (one word goes around another one).
'mozzarella but' becomes 'mat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'not' becomes 'o' (I've seen this in other clues).
'in the middle' means one lot of letters goes inside another.
'to' going around 'mat' is 'tmato'.
'tmato' enclosing 'o' is 'TOMATO'.
Can you help me to learn more?
(Other definitions for tomato that I've seen before include "A motto, strangely, for this fruit" , "It's also known as a love apple" , "Red fruit eaten as veg" , "Shiny red fruit" , "Love apple (or "beef" fruit?)" .)