Warning marble bust will break completely (5-4)
I believe the answer is:
alarm bell
'warning' is the definition.
'marble bust will break completely' is the wordplay.
'bust' indicates an anagram.
'will break' indicates putting letters inside (inserted letters break into the word).
'completely' becomes 'all' (synonyms).
'marble' is an anagram of 'larmbe'.
'larmbe' inserted within 'all' is 'ALARM-BELL'.
(Other definitions for alarm bell that I've seen before include "Warning device" , "Warning sound" , "It rings as a warning" , "Warning signal" , "Ring it to signal danger" .)