Dead weight (5)
I believe the answer is:
plumb
This clue is a double definition.
'dead' is the first definition.
('plumb' can be a synonym of 'dead')
'weight' is the second definition.
(plumb is a kind of weight)
(Other definitions for plumb that I've seen before include "Lead weight; exactly" , "For Americans, quite" , "Check depth of water below" , "Lead weight for testing depth or the perpendicular" , "Weight to show vertical direction" .)