Street right through area of land — unknown quantity — it may never be restored (4,8)

'it may never be restored' is the definition.
(lost property might never be returned to its owner)

'street right through area of land unknown quantity' is the wordplay.
'street' becomes 'st' (abbreviation).
'right' becomes 'proper' (associated in meaning).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'area of land' becomes 'lot' (lot can mean a piece of land).
'unknown quantity' becomes 'y' (algebra).
'st'+'proper'='stproper'
'stproper' put inside 'lot' is 'lostpropert'.
'lostpropert'+'y'='LOST PROPERTY'