Legacy of millions missing from retreat (8)
I believe the answer is:
heritage
'legacy' is the definition.
(synonyms)
'millions missing from retreat' is the wordplay.
'millions' becomes 'm' (abbreviation).
'missing from' suggests deleting specific letters.
'retreat' becomes 'hermitage' (a hermitage may be a religious retreat).
'hermitage' with 'm' taken away is 'HERITAGE'.
'of' acts as a link.
(Other definitions for heritage that I've seen before include "Their age (anag.)" , "Birthright" , "Valued things passed down to us" , "Property or practices passed down" , "historical collection?" .)