Drum mostly filled with a fruit (6)
I believe the answer is:
tomato
'fruit' is the definition.
(type of fruit)
'drum mostly filled with a' is the wordplay.
'drum' becomes 'tomtom' (tom-tom is a kind of drum).
'mostly' means to remove the last letter (most but not all of the word).
'filled with' is an insertion indicator.
'tomtom' with its final letter removed is 'tomto'.
'tomto' placed around 'a' is 'TOMATO'.
(Other definitions for tomato that I've seen before include "Type of sauce" , "Part of a BLT sandwich" , "Love apple (archaic)" , "Pulpy edible fruit" , "Fruit, often used as vegetable" .)