Plump domestic, not quite sound (6)
I believe the answer is:
fathom
'sound' is the definition.
('fathom' can be a synonym of 'sound')
'plump domestic not quite' is the wordplay.
'plump' becomes 'fat' ('fat' can be a synonym of 'plump').
'domestic' becomes 'home' ('home' can be similar in meaning to 'domestic').
'not quite' means to remove the last letter.
'home' with its final letter removed is 'hom'.
'fat'+'hom'='FATHOM'
(Other definitions for fathom that I've seen before include "Work out" , "Unit of six feet in measuring depth of water" , "''Full ... five, thy father lies'' (The Tempest)" , "A measure of six feet in depth" , "downward measurement" .)