Counterweight (7)
I believe the answer is:
ballast
'counterweight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ballast that I've seen before include "Stabilising weight (naut.)" , "Heavy material taken aboard to steady ship" , "Heavy material giving ship stability" , "Material for railway bed" , "Steadying weight found at balls" .)