Covers ring with one large layer of earth (7)
I believe the answer is:
topsoil
'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'.
'tops'+'o'+'i'+'l'='TOPSOIL'
(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" .)