Boat stabilizer (7)
I believe the answer is:
ballast
'boat stabilizer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ballast that I've seen before include "Weight to steady ship" , "Steadying weight found at balls" , "Heavy material taken aboard to steady ship" , "Material put into a ship to improve its stability" , "Heavy material for steadying ship" .)