Book kept in PO Box for corporation (8)
I believe the answer is:
potbelly
'corporation' is the definition.
(I know that corporation can be written as potbelly)
'book kept in po box' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'kept in' means one lot of letters goes inside another.
'box' becomes 'telly' (both can mean a television set).
'po'+'telly'='potelly'
'b' put inside 'potelly' is 'POTBELLY'.
'for' is the link.
(Other definitions for potbelly that I've seen before include "Protruding stomach" , "Bulging stomach" , "Corporation" , "Slang term for a paunch" , "Spare tyre" .)