Gap sounds complete (4)
'gap' is the definition.
(hole is a kind of gap)
'sounds complete' is the wordplay.
'sounds' indicates a 'sounds like' (homophone) clue.
'complete' becomes 'whole' ('whole' can be similar in meaning to 'complete').
'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".)