Fruit mum used in toto (6)
I believe the answer is:
tomato
'fruit' is the definition.
(I've seen this before)
'mum used in toto' is the wordplay.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'used in' means one lot of letters goes inside another.
'ma' going within 'toto' is 'TOMATO'.
(Other definitions for tomato that I've seen before include "Love apple (archaic)" , "A motto turned out to be fruit/veg" , "Type of sauce" , "Red fruit eaten as veg" , "A glossy red fruit eaten as a vegetable" .)