Covers ring with one large layer of earth (7)
'layer of earth' is the definition.
'covers ring with one large' is the wordplay.
'covers' becomes 'tops' (topping is a kind of covering).
'ring' becomes 'o'.
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (roman numeral).
'large' becomes 'l'.
(Other definitions for topsoil that I've seen before include "The Good Earth", "Garden's surface layer", "where grass grows", "Visible layer of garden", "Upper garden layer".)