Gains by Labour picked up in ballot boxes (4)
I believe the answer is:
urns
'ballot boxes' is the definition.
(I've seen this before)
'gains by labour picked up' is the wordplay.
'gains by labour' becomes 'earns' (I've seen this before).
'picked up' shows a homophone (sound like).
'earns' sounds like 'URNS'.
'in' acts as a link.
(Other definitions for urns that I've seen before include "Vessels to hold the ashes of the dead" , "Grecian vases" , "Large vases or pots" , "Runs to get large vases" , "Vaselike vessels on pedestals" .)