Enlightened state strangely invariant without outsiders (7)
I believe the answer is:
nirvana
'enlightened state' is the definition.
'strangely invariant without outsiders' is the wordplay.
'strangely' indicates an anagram.
'without outsiders' means to look at the middle letters.
The middle of 'invariant' is 'nvarian'.
'nvarian' with letters rearranged gives 'NIRVANA'.
(Other definitions for nirvana that I've seen before include "religious state" , "religious ecstasy" , "Buddhists' transcendental state" , "Hindu or Buddhist blissful meditative state" , "The Buddhist's version of Paradise" .)