Round item a stone in coarse aggregate (7)
I believe the answer is:
ballast
'coarse aggregate' is the definition.
(coarse material used in concrete)
'round item a stone' is the wordplay.
'round item' becomes 'ball' (balls are round).
'stone' becomes 'st'.
'ball'+'a'+'st'='BALLAST'
'in' acts as a link.
(Other definitions for ballast that I've seen before include "Stabilising material" , "Heavy stuff used to stabilise boat" , "Heavy material giving ship stability" , "Mass giving ship stability" , "Weight to stabilise a ship" .)