Otto's upset about mum being in the soup (6)
I believe the answer is:
tomato
'the soup' is the definition.
(thesaurus)
'otto's upset about mum' is the wordplay.
'upset' is an anagram indicator.
'about' means one lot of letters goes inside another.
'mum' becomes 'ma' (both can mean mother).
'otto' with letters rearranged gives 'toto'.
'toto' going around 'ma' is 'TOMATO'.
'being in' is the link.
(Other definitions for tomato that I've seen before include "Love apple (or "beef" fruit?)" , "A motto, strangely, for this fruit" , "Fruit, often used as vegetable" , "that's suitable to eat" , "Salad fruit or berry" .)