Mike leaves retreat in background (8)
I believe the answer is:
heritage
'background' is the definition.
(as in someone's national or ethnic background)
'mike leaves retreat' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'leaves' suggests deleting specific letters.
'retreat' becomes 'hermitage' (a hermitage may be a religious retreat).
'hermitage' with 'm' removed is 'HERITAGE'.
'in' acts as a link.
(Other definitions for heritage that I've seen before include "Birth-related condition" , "Property or practices passed down" , "Received culture and background" , "Birthright" , "Our historic legacy" .)