Sounds like one gains by labour with water heaters (4)
I believe the answer is:
urns
'water heaters' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe they are linked in a way I don't understand?
'sounds like one gains by labour' is the wordplay.
'sounds like' shows a homophone (sound like).
'one gains by labour' becomes 'earns' (I've seen this before).
'earns' is a homophone of 'URNS'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for urns that I've seen before include "Runs to find large vases" , "Containers for ashes" , "Vaselike vessels on pedestals" , "Tall vases" , "Large pots for tea" .)