Part inside reactor now ruptured (4)
I believe the answer is:
torn
'ruptured' is the definition.
(thesaurus)
'part inside reactor now' is the wordplay.
'part' says the answer is hidden in the clue.
'TORN' is hidden in the letters of 'inside reactor now'.
(Other definitions for torn that I've seen before include "Ripped - undecided" , "'-- Curtain', 1966 Hitchcock film" , "In a state of uncertainty between two conflicting options" , "Having rips" , "Dance wear" .)