It's bad when there's a missing part (3)
I believe the answer is:
sin
'it's bad' is the definition.
(thesaurus)
'there's a missing part' is the wordplay.
'part' indicates a hidden word.
'SIN' can be found hidden inside 'theres missing'.
'when' is the link.
(Other definitions for sin that I've seen before include "Immoral action" , "Breach of moral law" , "Mortal error" , "Evil" , "Serious error" .)