Gap sounds complete (4)
I believe the answer is:
hole
'gap' is the definition.
(I know that hole is a type of gap)
'sounds complete' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'complete' becomes 'whole' (associated in meaning).
'whole' sounds like 'HOLE'.
(Other definitions for hole that I've seen before include "Gap; difficult situation" , "Aperture; flaw in a plan" , "mess" , "See 9" , "Well" .)