Horrible bra came undone (7)
I believe the answer is:
macabre
'horrible' is the definition.
(thesaurus)
'bra came undone' is the wordplay.
'undone' indicates an anagram (the original order is undone).
'bra'+'came'='bracame'
'bracame' with letters rearranged gives 'MACABRE'.
(Other definitions for macabre that I've seen before include "'Grisly, gruesome (7)'" , "Ghastly, gruesome" , "Brace ma, being gruesome" , "Chilling" , "gruesome result" .)