Counterweight (7)
I believe the answer is:
ballast
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">counterweight</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)