Part of reactor now ripped apart (4)
I believe the answer is:
torn
'ripped apart' is the definition.
(I've seen this in another clue)
'part of reactor now' is the wordplay.
'part of' indicates a hidden word.
'TORN' is hidden within 'reactor now'.
(Other definitions for torn that I've seen before include "Lacerated - in two minds" , "Removed forcibly" , "Having rips" , "Divided" , "In a state of uncertainty between two conflicting options" .)