Central rule can be broken (7)
I believe the answer is:
nuclear
'central' is the definition.
(similar in meaning)
'rule can be broken' is the wordplay.
'be broken' indicates an anagram.
'rule'+'can'='rulecan'
'rulecan' is an anagram of 'NUCLEAR'.
(Other definitions for nuclear that I've seen before include "and 6 Down: Power producer" , "Type of reaction" , "power source" , "winter, sort of" , "Relating to atomic energy" .)