Tube empty when picked up (4)
I believe the answer is:
vein
'tube' is the definition.
(vein is a kind of tube)
'empty when picked up' is the wordplay.
'empty' becomes 'vain' (I've seen this before).
'when picked up' shows a homophone (sound like).
'vain' sounds like 'VEIN'.
(Other definitions for vein that I've seen before include "Rock fissure or minerals therein" , "Seam of mineral" , "Distinctive style" , "Distinctive quality" , "Part of the blood circulation system" .)