See books covering county’s missing years? (4-4)
I believe the answer is:
Here is my best explanation:
'missing years?' is the definition.
'see books covering county's' is the subsidiary indication.
'see' becomes 'lo' (archaic word meaning 'look!').
'books' becomes 'nt' (New Testament).
'covering' indicates putting letters inside.
'county' becomes 'glos' (abbreviation for Gloucestershire).
'nt' enclosing 'glos' is 'nglost'.
(Another definition for long-lost that I've seen is "Missing for some time".)