Crack in ground, by the sound of it? (4)
I believe the answer is:
flaw
'crack' is the definition.
(thesaurus)
'ground by the sound of it?' is the wordplay.
'ground' becomes 'floor' (floor is a kind of ground).
'by the sound of it?' indicates a 'sounds like' (homophone) clue.
'floor' is a homophone of 'FLAW'.
'in' is the link.
(Other definitions for flaw that I've seen before include "Imperfection or blemish" , "(Make) defect(ive)" , "Error in theory or plan" , "weak spot" , "Weakness" .)