Dessert’s component, mostly just filling (7)
I believe the answer is:
parfait
'dessert's' is the definition.
(parfait is a kind of dessert)
'component mostly just filling' is the wordplay.
'component' becomes 'part' (a component is a part of something).
'mostly' means to remove the last letter.
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'filling' indicates putting letters inside.
'fair' with its final letter taken away is 'fai'.
'part' going around 'fai' is 'PARFAIT'.
(Other definitions for parfait that I've seen before include "Rich dessert" , "sweet" , "Escoffier's faultless" , "Fruit and cream dessert" , "Rich iced pudding of eggs and cream." .)