Stabilising weight (7)
I believe the answer is:
ballast
'stabilising weight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ballast that I've seen before include "Steadying weight found at balls" , "Weight to stabilise a ship" , "Heavy material giving ship stability" , "Heavy material taken aboard to steady ship" , "Heavy material for steadying ship" .)