Cord quietly breaks eggs (4)
I believe the answer is:
rope
'cord' is the definition.
(rope is a kind of cord)
'quietly breaks eggs' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'breaks' indicates putting letters inside (inserted letters break into the word).
'eggs' becomes 'roe' (fish eggs).
'p' going inside 'roe' is 'ROPE'.
(Other definitions for rope that I've seen before include "Guy" , "Maybe tied up in bonds" , "will it bring in easy money when old?" , "Noose" , "Length of material used to tie or secure things" .)