Prosper having kept shilling in chest (5)
I believe the answer is:
bosom
'chest' is the definition.
(I know that bosom is a type of chest)
'prosper having kept shilling' is the wordplay.
'prosper' becomes 'boom' (synonyms).
'having kept' indicates putting letters inside.
'shilling' becomes 's' (abbreviation e.g. Lsd - pounds, shillings, pence).
'boom' enclosing 's' is 'BOSOM'.
'in' is the link.
(Other definitions for bosom that I've seen before include "Woman's breast" , "Woman's chest" , "This friend is dear and close" , "Female breasts" , "Intimate" .)