Fine deposit: smaller amount covers nothing (5)
I believe the answer is:
loess
'fine deposit' is the definition.
(sediment deposited by wind)
'smaller amount covers nothing' is the wordplay.
'smaller amount' becomes 'less' (I've seen this before).
'covers' is an insertion indicator.
'nothing' becomes 'O' (O resembles 0).
'less' going around 'o' is 'LOESS'.
(Other definitions for loess that I've seen before include "Compacted wind-blown sediment (geology)" , "Silt deposit" , "geological feature" , "Wind-blown loamy deposit" , "desposit" .)