Sounds like complete mess (4)
I believe the answer is:
hole
'mess' is the definition.
('hole' can be a synonym of 'mess')
'sounds like complete' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'complete' becomes 'whole' ('whole' can be similar in meaning to 'complete').
'whole' is a homophone of 'HOLE'.
(Other definitions for hole that I've seen before include "See 9" , "This in one is a golfer's dream!" , "Nasty place" , "Empty space; aperture" , "Hollow, cavity" .)