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 "Red fruit or vegetable", "A motto, strangely, for this fruit", "A motto, oddly, for this fruit", "Salad fruit or berry", "A glossy red fruit eaten as a vegetable", "Fruit eaten as a vegetable", "Fruit used in salads", ""Plum" or "beef" fruit?", "Is it a fruit or vegetable?" and "Pulpy fruit eaten as vegetable".)