Fruit mum used in toto (6)
I believe the answer is:
Here is my best explanation:
'fruit' is the definition.
(I've seen this before)
'mum used in toto' is the subsidiary indication.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'used in' indicates putting letters inside.
'ma' put into 'toto' is 'tomato'.
(Other definitions for tomato that I've seen before include "It's also known as a love apple", "Is it a fruit or vegetable?", "Love apple (archaic)", "Pulpy fruit eaten as vegetable", "A motto turned out to be fruit or veg", "A glossy red fruit eaten as a vegetable", ""Plum" or "beef" fruit?", "Fruit eaten as a vegetable", "In France, it's a love apple" and "Salad plant".)